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Chapter 4 Lenses and Optics

CONTENTS

4.1 Overview 4.2 Lens Functions and Properties

4.2.1 Focal Length and Field of View 4.2.1.1 Field-of-View Calculations

4.2.1.1.1 Tables for Scene Sizes vs. FL for 1/4-, 1/3-, and 1/2-Inch Sensors

4.2.1.1.2 Tables for Angular FOV vs. FL for 1/4-, 1/3-, and 1/2-Inch Sensor Sizes

4.2.1.2 Lens and Sensor Formats 4.2.2 Magnification

4.2.2.1 Lens–Camera Sensor Magnification 4.2.2.2 Monitor Magnification 4.2.2.3 Combined Camera and Monitor

Magnification 4.2.3 Calculating the Scene Size

4.2.3.1 Converting One Format to Another 4.2.4 Calculating Angular FOV 4.2.5 Lens Finder Kit 4.2.6 Optical Speed: f-number 4.2.7 Depth of Field 4.2.8 Manual and Automatic Iris

4.2.8.1 Manual Iris 4.2.8.2 Automatic-Iris Operation

4.2.9 Auto-Focus Lens 4.2.10 Stabilized Lens

4.3 Fixed Focal Length Lens 4.3.1 Wide-Angle Viewing 4.3.2 Narrow-Angle Telephoto Viewing

4.4 Vari-Focal Lens 4.5 Zoom Lens

4.5.1 Zooming 4.5.2 Lens Operation 4.5.3 Optical Speed 4.5.4 Configurations 4.5.5 Manual or Motorized

4.5.6 Adding a Pan/Tilt Mechanism 4.5.7 Preset Zoom and Focus 4.5.8 Electrical Connections 4.5.9 Initial Lens Focusing

4.5.10 Zoom Pinhole Lens 4.5.11 Zoom Lens–Camera Module 4.5.12 Zoom Lens Checklist

4.6 Pinhole Lens 4.6.1 Generic Pinhole Types 4.6.2 Sprinkler Head Pinhole 4.6.3 Mini-Pinhole

4.7 Special Lenses 4.7.1 Panoramic Lens—360�

4.7.2 Fiber-Optic and Bore Scope Optics 4.7.3 Bi-Focal, Tri-Focal Image Splitting Optics 4.7.4 Right-Angle Lens 4.7.5 Relay Lens

4.8 Comments, Checklist and Questions 4.9 Summary

4.1 OVERVIEW

The function of the camera lens is to collect the reflected light from a scene and focus it onto a camera sensor. Choosing the proper lens is very important, since its choice determines the amount of light received by the camera sensor, the FOV on the monitor, and the quality of the image displayed. Understanding the characteristics of the lenses available and following a step-by-step design proce- dure simplifies the task and ensures an optimum design.

A CCTV lens functions like the human eye. Both col- lect light reflected from a scene or emitted by a lumi- nous light source and focus the object scene onto some receptor—the retina or the camera sensor. The human eye has a fixed-focal-length (FFL) lens and variable iris

72 CCTV Surveillance

diaphragm, which compares to an FFL, automatic-iris video lens. The eye has an iris that opens and closes just like an automatic-iris camera lens and automatically adapts to changes in light level. The iris—whether in the eye or in the camera—optimizes the light level reaching the recep- tor, thereby providing the best possible image. The iris in the eye is a muscle-controlled membrane; the automatic iris in a video lens is a motorized device.

Of the many different kinds of lenses used in video secu- rity applications the most common is the FFL lens, which is available in wide-angle (90�), medium-angle (40�), and narrow-angle (5�) FOVs. To cover a wide scene and also obtain a close-up (telephoto) view with the same camera, a variable-FOV vari-focal or zoom lens is used. The vari- focal lens is used to “fine tune” the focal length (FL) to a specific FL for the application. To further increase the camera’s FOV a zoom lens mounted on a pan/tilt platform is used.

The pinhole lens is used for covert video surveillance applications since it has a small front diameter and can easily be hidden. There are many other specialty lenses, including split-image, fiber optic, right-angle, and auto- matic focus.

A relatively new lens—the panoramic 360� lens—is used to obtain a 360� horizontal by up to 90� vertical FOV. This

lens must be used with a digital computer and software algorithm to make use of the donut-shaped image it pro- duces on the camera sensor. The software converts the image to a 360� panoramic display.

4.2 LENS FUNCTIONS AND PROPERTIES

A lens focuses an image of the scene onto the CCTV cam- era sensor (Figure 4-1). The sensor can be a CCD, CMOS, ICCD, or thermal IR imager.

The lens in a human and a camera have some similar- ities: they both collect light and focus it onto a receptor (Figure 4-2).

They have one important difference: the human lens has one FFL and the retina is one size, but the camera lens may have many different FLs and the sensor may have different sizes. The unaided human eye is limited to seeing a fixed and constant FOV, whereas the video system can be modified to obtain a range of FOVs. The eye has an automatic-iris diaphragm to optimize the light level reaching the retina. The camera lens has an iris (either manual or automatic) to control the light level reaching the sensor (Figure 4-3).

HEIGHT

LENS FIELD

OF VIEW (FOV)

C OR CS

VIDEO OUT

POWER IN

V = VERTICAL HEIGHT

H = HORIZONTAL WIDTH

D = DIAGONAL

H × V = CAMERA SENSOR FOV

REFLECTED LIGHT FROM SENSOR

NATURAL OR ARTIFICIAL ILLUMINATION SOURCE

CAMERA

WIDTH

MOUNT

SCENE VIEWED BY CAMERA /LENS

CCD, CMOS, IR SENSOR

LENS

FIGURE 4-1 CCTV camera/lens, scene, and source illumination

Lenses and Optics 73

CAMERA SENSOR FIELD OF VIEW (FOV)

SCENE

EYE FIELD OF VIEW

EYE MAGNIFICATION = 1

EYE LENS FOCAL LENGTH ≈ 17 mm (0.67")

EYE RETINA

CAMERA SENSOR

LENS

IRIS

17 mm

SENSOR FORMAT

1/2" 1/3"

SCENE

1/4"

2/3"

FIGURE 4-2 Comparing the human eye to a CCTV lens and camera sensor

EYE AUTOMATIC IRIS

IRIS ALMOST CLOSED WHEN VIEWING BRIGHT SCENE (SUN)

IRIS HALF CLOSED WHEN VIEWING NORMAL SCENE (INDOORS)

IRIS WIDE OPEN WHEN VEIWING DARK SCENE (NIGHT TIME)

CCTV LENS AUTOMATIC IRIS

MOTOR DRIVEN IRIS

DRIVE MOTOR/GEAR

MOTOR VIDEO SIGNAL

METAL LEAVES OPEN AND CLOSE BY MOVING LENS IRIS RING

IRIS OPEN

HALF CLOSED

IRIS NEARLY CLOSED

DRIVE

FIGURE 4-3 Comparing the human eye and CCTV camera lens iris

74 CCTV Surveillance

4.2.1 Focal Length and Field of View

In the human eye, magnification and FOV are set by the lens FL and retina size. When the human eye and the video camera lens and sensor see the same basic picture, they are said to have the same FOV and magnification. In practice, a lens that has an FL and FOV similar to that of the human eye is referred to as a normal lens with a magnification M = 1. The human eye’s focal length—the distance from the center of the lens at the front of the eye to the retina in the back of the eye—is about 17 mm (0.67 inch) (Figure 4-2).

Most people see approximately the same FOV and mag- nification (M = 1). Specifically, the video lens and cam- era format corresponding to the M = 1 condition is a 25 mm FL lens on a 1-inch (diagonal) format camera, a 16 mm lens on a 2/3-inch format camera, a 12.5 mm lens on a 1/2-inch camera, an 8 mm lens on a 1/3-inch cam- era, and a 6 mm lens on a 1/4-inch sensor. The 1-inch format designation was derived from the development of the original vidicon television tube, which had a nom- inal tube diameter of 1 inch (25.4 mm) and an actual scanned area (active sensor size) of approximately 16 mm in diameter. Figure 4-4 shows the FOV as seen with a lens having magnifications of 1, 3, and 1/3 respectively.

Lenses with much shorter FL used with these sensors are referred to as wide-angle lenses and lenses with much longer FL are referred to as narrow-angle (telephoto) lens. Between these two are medium FL lenses. Telephoto lenses used with video cameras act like a telescope: they magnify the image viewed, narrow the FOV, and effectively bring the object of interest closer to the eye. While there is no device similar to the telescope for the wide-angle example, if there were, the device would broaden the FOV, allowing the eye to see a wider scene than is normal and at the same time causing objects to appear farther away from the eye. One can see this condition when looking through a telescope backwards. This also occurs with the automobile passenger side-view mirror, a concave mirror that causes the scene image to appear farther away, and therefore smaller than it actually is (de-magnified).

Just as your own eyes have a specific FOV—the scene you can see—so does the video camera. The camera FOV is determined by the simple geometry shown in Figure 4-5.

The scene has a width (W ) and a height (H ) and is at a distance (D) away from the camera lens. Once the scene has been chosen, three factors determine the correct FL lens to use: (1) the size of the scene (H� W ), (2) the distance between the scene and camera lens (D), and (3) the camera image sensor size (1/4-, 1/3-, or 1/2-inch format).

NORMAL M = 1

WIDE ANGLE M = 1/3

NARROW ANGLE M = 3

MONITOR

FIGURE 4-4 Lens FOV for magnifications of 3, 1, and 1/3

Lenses and Optics 75

DIAGONAL(d ) HORIZONTAL(h) VERTICAL(v )

11 0.43 8.8 0.35 6.6 0.26

FORMAT

2/3"

1/2" 8 0.31 6.4 0.25 4.8 0.19

6 0.24 4.8 0.19 3.6 0.141/3"

inchmm mm mm inchinch

4 0.16 3.2 0.13 2.4 0.11/4"

1" 16 0.63 12.8 0.50 9.6 0.38

1/6" 3 0.12 2.4 0.09 1.8 0.07

LENS

VERTICAL HEIGHT

D = DISTANCE FROM SCENE TO LENS

HORIZONTAL WIDTH

CAMERA SENSOR FOV

SOLID STATE (CCD)

CAMERA

SENSOR SENSOR GEOMETRY

CCTV

SCENE

TUBE

d = DIAGONAL

W H

VERTICAL = 3 UNITS HIGH

HORIZONTAL = 4 UNITS WIDE

v h

v = VERTICAL

h = HORIZONTAL

SENSOR SIZE

FIGURE 4-5 Camera/lens sensor geometry and formats

4.2.1.1 Field-of-View Calculations

There are many tables, graphs, monographs, and linear and circular slide rules for determining the angles and sizes of a scene viewed at varying distances by a video camera with a given sensor format and FL lens. One conve- nient aid in the form of transparent circular scales, called a “Lens Finder Kit,” eliminates the calculations required to choose a video camera lens (Section 4.2.5). Such kits are based on the simple geometry shown in Figure 4-6.

Since light travels in straight lines, the action of a lens can be drawn on paper and easily understood. Bear in mind that while commercial video lenses are constructed from multiple lens elements, the single lens shown in Figure 4-6 for the purpose of calculation has the same effective FL as the video lens. By simple geometry, the scene size viewed by the sensor is inversely proportional to the lens FL. Shown in Figure 4-6 is a camera sensor of hor- izontal width (h) and vertical height (v). For a 1/2-inch CCD sensor, this would correspond to h = 6�4 mm and v = 4�8 mm. The lens FL is the distance behind the lens at which the image of a distant object (scene) would focus. The figure shows the projected area of the sensor on the scene at some distance D from the lens. Using the eye analogy, the sensor and lens project a scene W wide × H high (the eye sees a circle as did the original vidicon). As with the human eye, the video lens inverts the image, but

the human brain and the electronics re-inverts the image in the camera to provide an upright image. Figure 4-6 shows how to measure or calculate the scene size (W × H ) as detected by a rectangular video sensor format and lens with horizontal and vertical angular FOVs �H and �V , respectively.

4.2.1.1.1 Tables for Scene Sizes vs. FL for 1/4-, 1/3-, and 1/2-Inch Sensors

Tables 4-1, 4-2, and 4-3 give scene-size values for the 1/4-, 1/3-, and 1/2-inch sensors, respectively, as a function of the distance from the camera to the object and the lens FL. The tables include scene sizes for most available lenses ranging from 2.1 to 150 mm FL.

To find the horizontal FOV �H, we use the geometry of similar triangles:

W = FL

D � W = h

FL × D (4-1)

The horizontal angular FOV �H is then derived as follows:

tan �H 2

= h/2 FL

�H 2

= tan−1 h 2 FL

�H = 2 tan−1 h

2 FL (4-2)

76 CCTV Surveillance

θV = VERTICAL ANGLE OF VIEW

SCENE

FIXED FOCAL LENGTH LENS

CAMERA SENSOR

v = SENSOR VERTICAL HEIGHT h = SENSOR HORIZONTAL WIDTH

H = SCENE HEIGHT

W = SCENE WIDTH

CAMERA LOCATION SCENE LOCATION

θH = HORIZONTAL ANGLE OF VIEW

θH/2

θV/2

FIGURE 4-6 Sensor, lens and scene geometry

1/4 - INCH SENSOR FORMAT LENS GUIDE CAMERA TO SCENE DISTANCE (D) IN FEET

WIDTH AND HEIGHT OF AREA (W × H ) IN FEET 5 10 20 30 40 50 75 100

LENS FOCAL

LENGTH (mm)

ANGULAR FIELD OF

VIEW: H × V (DEG.) W × H W × H W × H W × H W × H W × H W × H W × H

2.1 81.2 × 60.9 8.6 × 6.4 17 × 12.9 34 × 26 51 × 39 69 × 51 86 × 64 129 × 96 171 × 129 2.2 78.6 × 59.0 8.2 × 6.1 16.4 × 12.2 33 × 25 49 × 37 65 × 49 82 × 63 123 × 92 164 × 123 2.3 76.1 × 57.1 7.8 × 5.9 15.6 × 11.8 31 × 23 47 × 35 62 × 47 78 × 59 117 × 86 157 × 117 2.6 69.4 × 52 6.9 × 5.2 13.9 × 10.4 28 × 21 42 × 31 55 × 42 69 × 52 104 × 78 138 × 104 3.0 61.9 × 46.4 6.0 × 4.5 12 × 9 24 × 18 36 × 27 48 × 36 60 × 45 90 × 68 120 × 90 3.6 53.1 × 39.8 5.0 × 3.8 10 × 7.5 20 × 15 30 × 23 40 × 30 50 × 38 75 × 57 100 × 76 3.8 50.7 × 38.0 4.7 × 3.6 9.5 × 7.1 19 × 14 28 × 21 38 × 28 47 × 36 71 × 54 94 × 72 4.0 48.5 × 36.4 4.5 × 3.4 9 × 6.8 18 × 14 27 × 20 36 × 27 45 × 34 68 × 51 90 × 68 4.3 45.4 × 34.1 4.2 × 3.1 8.4 × 6.3 16.7 × 12.5 25 × 19 33 × 25 42 × 31 63 × 47 84 × 62 6.0 33.4 × 25.0 3 × 2.3 6 × 4.5 12 × 9 18 × 13.5 24 × 18 30 × 23 45 × 35 60 × 46 8.0 25.4 × 19.0 2.3 × 1.7 4.5 × 3.4 9 × 6.8 13.5 × 10.1 18 × 13.5 23 × 17 35 × 26 46 × 34

12.0 17.1 × 12.8 1.5 × 1.1 3 × 2.2 6 × 4.4 9.0 × 6.8 12 × 9 15 × 11 23 × 17 30 × 23 16.0 12.8 × 9.6 1.1 × .8 2.3 × 1.7 4.5 × 3.4 6.8 × 5.1 9 × 6.8 11.2 × 8.4 17 × 13 22 × 17 25.0 8.2 × 6.2 .72 × .54 1.4 × 1.1 2.9 × 2.1 4.3 × 3.2 5.8 × 4.3 7.2 × 5.4 10.8 × 8.1 14.4 × 10.8

NOTE: 1/4 - INCH LENSES ARE DESIGNED FOR 1/4 - INCH SENSOR FORMATS ONLY AND WILLNOT WORK ON 1/3 - INCH OR 1/2 - INCH SENSORS. LENS FOCAL LENGTHS ARE NOMINAL PER MANUFACTURERS’ LITERATURE. ANGULAR FOV AND W × H ARE DERIVED FROM EQUATIONS 4 - 1 TO 4 - 4 AND VERTICAL FOV FROM STANDARD 4:3 MONITOR RATIO: V = 0.75H.

Table 4-1 1/4-Inch Sensor FOV and Scene Sizes vs. FL and Camera-to-Scene Distance

Lenses and Optics 77

1/3-INCH SENSOR FORMAT LENS GUIDE

5 10 20 30 40 50 75 100

LENS FOCAL

LENGTH (mm)

ANGULAR FIELD OF VIEW: H × V

(DEG.) W × H W × H W × H W × H W × H W × H W × H W × H

92.4 × 69.3 10.4 × 7.8 20.8 × 15.6 41.6 ×3 1.2 63 × 47 83 × 62 104 × 78 156 × 117 208 × 156

85.4 × 64.1 9.2 × 6.9 18.5 × 13.8 36.8 × 27.6 55 × 41 77 × 58 92 × 69 138 × 104 184 × 138 81.2 × 60.9 8.6 × 6.5 17.2 × 13 34.4 × 26 51 × 39 69 × 52 86 × 65 129 × 98 172 × 130 67.4 × 50.5 6.7 × 5.0 13.3 × 10 26.7 × 20 40 × 30 53 × 40 67 × 50 101 × 75 134 × 100 64.6 × 48.4 6.3 × 4.7 12.6 × 9.5 25 × 18.9 37.9 × 28.4 50.5 × 37.9 63 × 47 95 × 71 123 × 92 61.9 × 46.4 6.0 × 4.5 12 × 9 24 × 18 36 × 27 48 × 36 60 × 45 90 × 68 120 × 90 56.1 × 42.1 5.3 × 4.0 10.6 × 8 21.2 × 15.9 31.8 × 23.9 42.4 × 31.8 53 × 40 80 × 60 106 × 80 43.6 × 32.7 4.0 × 3.0 8.0 × 6 16 × 12 24 × 18 32 × 24 40 × 30 60 × 45 80 × 60 33.4 × 25.0 3.0 × 2.3 6 × 4.5 12 × 9 18 × 13.5 24 × 18 30 × 22.5 45 × 34 60 × 45 26.6 × 20.0 2.0 × 1.5 4.0 × 3.0 8.0 × 6.0 12.0 × 9.0 16 × 12 20 × 15 30 × 23 40 × 30 17.1 × 12.8 1.5 × 1.2 3.0 × 2.3 6.0 × 4.5 9.0 × 6.8 12.0 × 9.0 15.0 × 11.3 23 × 17 30 × 22.5 11.0 × 8.2 .96 × .72 1.9 × 1.4 3.8 × 2.9 5.8 × 4.4 7.7 × 5.8 9.6 × 7.2 14.4 × 10.8 19.2 × 14.4

5.5 × 4.1 .48 × .36 .96 × .72 1.9 × 1.4 2.9 × 2.2 3.8 × 2.8 4.8 × 3.6 7.2 × 5.4 9.6 × 7.2

2.3 2.6 2.8 3.6 3.8 4.0 4.5 6.0 8.0

12.0 16.0 25.0 50.0 75.0 3.7 × 2.8 .32 × .24 .64 × .50 1.3 × .96 1.9 × 1.4 2.6 × 1.9 3.2 × 2.4 4.8 × 3.6 6.4 × 4.8

NOTE: MOST 1/3 - INCH LENSES WILL NOT WORK ON 1/2 - INCH SENSORS BUT ALL WILL WORK ON ALL 1/4 - INCH SENSORS. LENS FOCAL LENGTHS ARE NOMINAL PER MANUFACTURERS’ LITERATURE. ANGULAR FOV AND W × H ARE DERIVED FROM EQUATIONS 4 - 1 TO 4 - 4 AND VERTICAL FOV FROM STANDARD 4:3 MONITOR RATIO: V = 0.75H.

CAMERA TO SCENE DISTANCE (D) IN FEET WIDTH AND HEIGHT OF AREA (W × H ) IN FEET

Table 4-2 1/3-Inch Sensor FOV and Scene Sizes vs. FL and Camera-to-Scene Distance

1/2-INCH SENSOR FORMAT LENS GUIDE

CAMERA TO SCENE DISTANCE (D) IN FEET WIDTH AND HEIGHT OF AREA (W × H ) IN FEET

5 10 20 30 40 50 75 100

LENS FOCAL

LENGTH (mm)

ANGULAR FIELD OF

VIEW: H × V (DEG.) W × H W × H W × H W × H W × H W × H W × H W × H

1.4 133 × 100 23 × 17 46 × 34 91 × 69 137 × 103 183 × 137 228 × 171 342 × 257 457 × 348 2.6 101.8 × 76.4 12.3 × 9.2 24.6 × 18 49 × 37 74 × 55 98 × 74 123 × 92 185 × 138 246 × 184 3.5 84.9 × 63.7 9.1 × 6.9 18.2 × 13.8 37 × 28 55 × 41 73 × 55 91 × 69 137 × 104 182 × 138 3.6 83.3 × 62.5 8.9 × 6.7 17.8 × 13.4 36 × 27 53 × 40 71 × 53 89 × 67 134 × 101 178 × 134 3.7 81.7 × 61.3 8.6 × 6.5 17.2 × 13.0 35 × 26 52 × 39 69 × 52 86 × 65 129 × 98 172 × 130 4.0 77.3 × 58.0 8.0 × 6.0 16.0 × 12.0 32 × 24 48 × 36 64 × 24 80 × 60 120 × 90 160 × 120 4.2 74.6 × 56.0 7.6 × 5.7 15.2 × 11.4 30 × 23 48 × 34 61 × 46 76 × 57 114 × 86 156 × 114 4.5 70.8 × 53.1 7.1 × 5.3 14.2 × 10.6 28 × 21 43 × 32 57 × 43 71 × 53 107 × 80 142 × 107 4.8 67.4 × 50.5 6.7 × 5.0 13.4 × 10.0 27 × 20 40 × 30 53 × 40 67 × 50 101 × 75 134 × 100 6.0 56.1 × 42.1 5.3 × 4.0 10.6 × 8.0 21 × 16 32 × 24 43 × 32 53 × 40 80 × 60 106 × 80 7.5 46.2 × 34.7 4.3 × 3.2 8.6 × 6.4 17.1 × 12.8 26 × 19 34 × 26 43 × 32 65 × 48 86 × 64 8.0 43.6 × 32.7 4.0 × 3.0 8.0 × 6.0 16 × 12 24 × 18 32 × 24 40 × 30 60 × 45 80 × 60

12.0 29.9 × 22.4 2.7 × 2.0 5.3 × 4.0 10.7 × 8 16 × 12 21.3 × 16 27 × 20 41 × 30 53 × 40 16.0 22.6 × 17.0 2.0 × 1.5 4.0 × 1.5 8 × 6 12 × 9 16 × 12 20 × 15 30 × 23 40 × 30 25.0 14.6 × 10.9 1.3 × 1.0 2.6 × 2.0 5.1 × 3.8 7.7 × 5.8 10.2 × 7.7 12.8 × 9.6 19 × 14 25.6 × 19.2 50.0 7.3 × 5.5 .64 × .48 1.3 × 1.0 2.6 × 1.9 3.8 × 2.9 5.1 × 3.8 6.4 × 4.8 6.5 × 4.8 12.8 × 9.6 75.0 4.9 × 3.7 .43 × .32 .85 × .64 1.7 × 1.3 2.6 × 1.9 3.4 × 2.6 4.3 × 3.2 3.2 × 2.4 8.6 × 6.4

150.0 2.4 × 1.8 .21 × .16 .43 × .32 .85 × .64 1.3 × .96 1.7 × 1.3 2.1 × 1.6 9.6 × 7.2 4.3 × 3.2

NOTE: ALL 1/2-INCH FORMAT LENSES WILL WORK ON 1/3- AND 1/4 -INCH SENSORS. LENS FOCAL LENGTHS ARE NOMINAL PER MANUFACTURERS’ LITERATURE. ANGULAR FOV AND W × H ARE DERIVED FROM EQUATIONS 4-1 TO 4-4.

Table 4-3 1/2-Inch Sensor FOV and Scene Sizes vs. FL and Camera-to-Scene Distance

78 CCTV Surveillance

For the vertical FOV, similar triangles give:

H = FL

D � H = v

FL × D (4-3)

The vertical angular FOV �V is then derived from the geometry:

tan �v 2

= v/2 FL

�v 2

= tan−1 v 2 FL

�v = 2 tan−1 v

2 FL (4-4)

4.2.1.1.2 Tables for Angular FOV vs. FL for 1/4-, 1/3-, and 1/2-Inch Sensor Sizes

Table 4-4 shows the angular FOV obtainable with 1/4 -, 1/3-, 1/2-, and 2/3-inch sensors with some standard lenses from 1.4 to 150 mm FL. The values of angular FOV in Table 4-4 can be calculated from Equations 4-2 and 4-4.

4.2.1.2 Lens and Sensor Formats

Fixed focal length lenses must be used with either the image sensor size (format) for which they were designed or with a smaller sensor size. They cannot be used with larger sensor sizes because unacceptable image distortion and image darkening (vignetting) at the edges of the image occurs. When a lens manufacturer lists a lens for a 1/3-inch sensor format, it can be used on a 1/4-inch sensor but not on a 1/2-inch sensor without producing image vignetting. This problem of incorrect lens choice for a given format size occurs most often when a C or CS mount 1/3-inch format lens is incorrectly used on a 1/2-inch format camera. Since the lens manufacturer does not “over design” the lens, that is, make glass lens element diameters larger than necessary, check the manufacturer’s specifications for proper choice.

4.2.2 Magnification

The overall magnification from a specific camera, lens, and monitor depends on three factors: (1) lens FL, (2) camera sensor format, and (3) the monitor size (diag- onal). Video magnification is analogous to film magnifica- tion: the sensor is equivalent to the film negative, and the monitor is equivalent to the photo print.

4.2.2.1 Lens–Camera Sensor Magnification

The combination of the lens FL and the camera sensor size defines the magnification Ms at the camera location. For a specific camera, the sensor size is fixed. Therefore, no matter how large the image from the lens is at the sensor, the camera will see only as much of the image as

will fit onto the sensor. Lens magnification is measured relative to the eye which is defined as a normal lens. The eye has approximately a 17-mm FL and is equivalent to a 25-mm FL lens on a 1-inch format camera sensor.

Therefore, the magnification of a 1-inch (16-mm for- mat) sensor is

Ms = Lens focal length (mm) Sensor diagonal (mm)

Ms �1 inch� = FL

16 mm

(4-5)

For 2/3 inch (11-mm format) the magnification is

Ms �2/3 inch� = FL

11 mm (4-6)

For 1/2 inch (8-mm format) the magnification is

Ms �1/2 inch� = FL

8 mm (4-7)

For 1/3 inch (5.5-mm format) the magnification is

Ms �1/3 inch� = FL

5.5 mm (4-8)

For 1/4 inch (4-mm format) the magnification is

Ms �1/4 inch� = FL

4 mm (4-9)

Example: From Equation 4-7, a 16-mm FL lens on a 1 2 -inch

format camera would have a magnification of

Ms �1/2 inch� = FL

8 mm = 16 mm

8 mm = 2

4.2.2.2 Monitor Magnification

When the camera image is displayed on the CCTV mon- itor, a further magnification of the object scene takes place. The monitor magnification Mm is equivalent to the ratio of the monitor diagonal (dm) to the sensor diagonal (ds) or

M�monitor� = Mm = dm ds

(4-10)

Example: From Equation 4-10, for a 9-inch diagonal mon- itor (dm = 9 inches) and a 1/2 sensor format (ds = 8 mm = 0�315 inch)

Mm = 9

0�315 = 28�57

L en

ses an

d O

p tics

CAMERA ANGULAR FIELD OF VIEW (FOV) (DEGREES) 1/4 INCH SENSOR 1/3 INCH SENSOR 1/2 INCH SENSOR 2/3 INCH SENSOR

LENS FOCAL

LENGTH (mm)

MAXIMUM IMAGE

FORMAT

OPTICAL SPEED:

f/#

LENS MOUNT TYPE

HORIZONTAL VERTICAL HORIZONTAL VERTICAL HORIZONTAL VERTICAL HORIZONTAL VERTICAL

1.4 1/2 1.4 CS 101 76 135 101 180 135 2.1 1/4 1.0 CS 91 70 2.2 1/3 1.2 CS 93 69 2.3 1/3 1.4 CS 89 67 113 85 2.6 1/2 1.6 CS 72 54 100 75 128 96 2.8 1/3 1.2 CS 71 53 96 72 3.0 1/4 1.0 CS 65 49 3.5 1/2 1.4 CS, C 59 44 78 59 104 78 3.6 1/2 1/6 CS, C 54 41 72 54 93 71 3.7 1/2 1.6 CS 53 40 71 53 94 70 3.8 1/3 1.4 CS 51 39 68 51 4.0 1/2 1.2 CS 50 37 65 50 89 67 4.2 1/2

1/4 1.6 CS 49 36 64 49 87 65

4.3 1.4 CS 42 35 4.5 1/2 1.4 CS, C 44 34 59 45 79 59 4.8 1/2 1.4 CS, C 39 29 52 39 69 52 96 74 6.0 1/2 1.0 CS 33 25 57 43 57 43 7.5 2/3

2/3 1.4 CS, C 26 20 35 26 46 35

8.0 1.2 CS 25 19 33 25 45 34 58 45 12.0 2/3 1.2 CS, C 28 13 24 28 30 23 39 29 16.0 2/3 1.4 CS, C 13 10 17 13 22 17 31 23 25.0 2/3 1.4 CS, C 8 6 12 9 15 11 20 15 50.0 2/3 1.4 CS, C 4.1 3.1 5.5 4.1 7.3 5.5 10 7.5 75.0 2/3 1.4 CS, C 2.8 2.1 3.7 2.8 4.8 3.6 6.8 5

150.0 1/2 1.6 CS, C 1.4 1.1 1.8 1.4 2.4 1.8 3.3 2.5

NOTE: ALL FOCAL LENGTHS AND ANGULAR FOVs BASED ON MANUFACTURER’S LITERATURE. ALL THE LARGER FORMAT LENSES CAN BE USED ON SMALLER FORMAT SENSORS. LENSES ARE ALSO AVAILABLE HAVING SMALLER FORMATS AND LOWER f/#S THAN THOSE LISTED.

Table 4-4 Representative Fixed Lenses Angular FOV vs. Sensor Format and Lens Focal Length

80 CCTV Surveillance

4.2.2.3 Combined Camera and Monitor

The combined lens, sensor, and monitor magnification is

M = Ms × Mm For the example above and Equation 4-11, the overall mag- nification of the 8-mm FL lens, 1/2-inch format camera, and a 9-inch monitor is

M = Ms × Mm = 2 × 28�57 = 57�14 Table 4-5 summarizes the magnification for the entire video system, for a 9- and 17-inch monitor and various lenses and camera formats. It should be noted that increas- ing the magnification by using a larger monitor does not increase the information in the scene; it only increases the size of the displayed picture and permits viewing the monitor from a greater distance.

4.2.3 Calculating the Scene Size

Equations 4-1 and 4-3 are used to calculate scene size. For example, calculate the horizontal and vertical scene size as seen by a 1/2-inch CCD sensor using a 12.5 mm FL lens at a distance D = 25 ft. A 1/2-inch sensor is 6.4 mm wide and 4.8 mm high. From Equation 4-1 for horizontal scene width:

Scene width = W = h FL

× D

W = 6.4 mm 12.5 mm

× 25 ft = 12.8 ft

For vertical scene height, using Equation 4-1:

Scene height = H = v FL

× D

H = 4.8 mm 12.5 mm

× 25 ft = 9.6 ft

4.2.3.1 Converting One Format to Another

To obtain scene sizes (width and height) for a 1/6-inch sensor, divide all the scene sizes in the 1/3-inch table (Table 4-2) by 2. For a 2/3-inch sensor, multiple all the scene sizes in the 1/3-inch table (Table 4-2) by 2.

Understanding Tables 4-1, 4-2, and 4-3 makes it easy to choose the right lens for the required FOV coverage. As an example, choose a lens for viewing all of a building 15 feet high by 20 feet long from a distance of 40 feet with a 1/2-inch format video camera (Figure 4-7). From Table 4-3, a 12-mm FL lens will just do the job.

If a 1/4-inch format video camera were used, a lens with an FL of 16 mm would be needed (from Table 4-4, a scene 16.7 feet high by 22.5 feet wide would be viewed).

If a 1/3-inch format video camera were used, a lens with an FL of 9 mm would be used (from Table 4-2, a scene 15.2 feet high by 20 feet wide would be viewed).

4.2.4 Calculating Angular FOV

Equations 4-2 and 4-4 are used to calculate the horizontal and vertical angular FOV of the lens–camera combination. Table 4-4 shows the angular FOV obtainable with some

1/6 (0.11/2.75)

1/4 (0.15/4.0)

1/3 (0.22/5.5)

1/2 (0.31/8.0)

9 72.7

TOTAL MAGNIFICATIONMONITOR SIZE (inch)

LENS FOCAL LENGTH mm

CAMERA FORMAT (inch/mm)

909.1

137.4

1717.2

37.3

358.3

70.4

28.7

54.0

17.1

267.8

32.4

506.0

712.2

377.0

676.8

2.4

2.6

25.0

3.8

4.8

75.0

4.8

75.0

50.0

2.4

ALL VALUES BASED ON SENSOR AND MONITOR DIAGONAL

EXAMPLE: 1/3-inch FORMAT SENSOR, 3.8 mm FL LENS (0.15 inch), AND 17-inch MONITOR

MAGNIFICATION = Ms × Mm, WHERE Ms = LENS FL

and, Mm = SENSOR DIAGONAL SENSOR DIAGONAL

MONITOR DIAGONAL

M = 3.8 mm 17 inch

0.22 inch = 54

5.5 mm ×

Table 4-5 Monitor Magnification vs. Camera/Monitor Size and Lens Focal Length

Lenses and Optics 81

LENS SENSOR

H = SCENE HEIGHT

LENS

SENSOR

W = SCENE WIDTH

SIDE VIEW

TOP(PLAN)

VIEW

ANGULAR WIDTH

θ H

ANGULAR WIDTH

θ V H = 22.5 ft

W = 30 fth

D = 40 ft

VERTICAL FOV: v/FL = H/D

HORIZONTAL FOV: h/FL = W/D

SCENE LOCATIONCAMERA LOCATION

FIGURE 4-7 Calculating the focal length for viewing a building

standard lenses from 2.6 to 75 mm focal length. For the previous example, calculate the horizontal and vertical angular FOVs �H and �V for a 1/2-inch CCD sensor using a 12.5 mm FL lens. The distance need not be supplied, since an angular measure is independent of distance.

From Equation 4-2, for horizontal angular FOV:

tan �H 2

= h/2 FL

= 6�6 mm/2 12�5 mm

= 0�264 �H 2

= 14�8�

�H = 29�6�

From Equation 4-4 for vertical angular FOV:

tan �v 2

= v/2 FL

= 4�8 mm/2 12�5 mm

= 0�192 �v 2

= 10�9�

�v = 21�8�

Table 4-4 summarizes angular FOV values for some stan- dard lenses from 1.4 to 150 mm FL lenses used on the 1/4-, 1/3-, 1/2- and 2/3-inch sensors. To obtain the angu- lar FOV for sensor sizes, multiply or divide the angles by the

ratio of the sensor size. Rule of thumb: for a given lens, angular FOV increases for larger sensor size, decreases for smaller sensor size.

4.2.5 Lens Finder Kit

Tables and slide rules for finding lens angular FOVs abound. Over the years many charts and devices have been available to simplify the task of choosing the best lens for a particular security application. Figure 4-8 shows how to quickly determine the correct lens for an application using the Lens Finder Kit (copyright H. Kruegle).

There is a separate scale for each of the three camera- sensor sizes: 1/4-, 1/3-, 1/2-inch (the 1/4- and 1/3-inch are shown). The scale for each camera format shows the FL of standard lenses and the corresponding angular hor- izontal and vertical FOVs that the camera will see.

To use the kit, the plastic disk is placed on the facility plan drawing and the lens FL giving the desired camera FOV coverage is chosen. For example, a 1/4-inch format camera is to view a horizontal FOV (�H) in a front lobby 30 feet wide at a distance of 30 feet from the camera (Figure 4-9). What FL lens should be used?

82 CCTV Surveillance

2. USING A SCALE DRAWING OF THE FLOOR PLAN (ANY SCALE), PLACE THE CENTER HOLE OF THE DISK AT THE PROPOSED CAMERA LOCATION ON THE FLOOR PLAN. 3. ROTATE THE DISK UNTIL ONE SEGMENT (PIE SECTION) TOTALLY INCLUDES THE HORIZONTAL FIELD OF VIEW REQUIRED. 4. USE THE FOCAL LENGTH LENS DESIGNATED IN THE SEGMENT ON THE DISK. 5. IF THE SCALE DRAWING INCLUDES AN ELEVATION VIEW, FOLLOW STEPS 1 THROUGH 4 AND USE THE VERTICAL ANGLE DESIGNATED IN EACH PIE SEGMENT FOR THE VERTICAL FIELD

OF VIEW OF THE LENS.

THE LENS FINDER KIT USES THREE TRANSPARENT PROTRACTOR DISKS TO HELP CHOOSE THE BEST LENS WHEN USING THE 1/4-, 1/3- AND 1/2-INCH CCTV CAMERA FORMATS WITH C OR CS MOUNTS. THE DISKS ARE UNIVERSAL AND CAN BE USED

1. SELECT THE DISK TO MATCH THE CAMERA FORMAT: 1/4-, 1/3- OR 1/2-INCH.

ON ANY SCALE DRAWING. HOW TO USE:

1/3 INCH FORMAT

1/4 INCH FORMAT

NOTE: FOR 2/3- AND 1/2-INCH FORMATS MULTIPLY THE 1/3- AND 1/4-INCH SCALE FOV'S BY 2

8 13

3 5

54 HORIZ

93 HORIZON

TAL

4 1

3.0 m

3.6mm

4.0 m

m 6

m m

8 m

12 m

m 16 mm

25 mm

1/4 "

SECURIT YW

O RK

AVID A

64 H

O RIZ

VER TIC

VERT.VE R

LENS FIN D

E R

2.2 m mC C T

718

548

488

348

248

17 8

13 8 8

3.6 m

4.0 m m

6. 0

m m

16 mm

25 mm

1/3 "

728 H O

RIZ

648 HOR IZ

11 948 HORIZON

TAL

SECU RI

TY W

O R

K S

ID A

V E

R T.

V ERT.

VER TIC

LENS FIN D

E R

2.8 m m

C C T V

FIGURE 4-8 Choosing a lens with the Lens Finder Kit

To find the horizontal angular FOV �H, draw the follow- ing lines to scale on the plan: one line to a distance 30 feet from the camera to the center of the scene to be viewed, a line 30 feet long and perpendicular to the first line, and two lines from the camera location to the endpoints of the second 30-foot line. Place the 1/4-inch Lens Finder Kit on the top view (plan) drawing with its center at the camera location and choose the FL closest to the horizon- tal angle required. A 3.6 mm FL lens is closest. This lens will see a horizontal scene width of 30 feet. Likewise for scene height: using the side-view (elevation) drawing, the horizontal scene height is 22.5 feet.

4.2.6 Optical Speed: f-number

The optical speed or f-number (f/#) of a lens defines its light-gathering ability. The optical speed of a lens—how much light it collects and transmits to the camera sensor— is defined by a parameter called the f-number (f/#).

As the FL of a lens becomes longer, its optical aperture or diameter (d ) must increase proportionally to keep the f-number the same. The f-number is related to the FL and the lens diameter (clear aperture) d by the following equation:

f/# = FL d

(4-11)

For example, an f/2.0 lens transmits four times as much light as an f/4.0 lens. The f-number relationship is analo- gous to water flowing through a pipe. If the pipe diameter is doubled, four times as much water flows through it. Likewise, if the f-number is halved (i.e. if the lens diameter is doubled), four times as much light will be transmitted through the lens.

In practice the f-number obtained is worse than this because of various losses caused by imperfect lens transmis- sion, reflection, absorption, and other lens imaging prop- erties. The amount of light (I ) collected and transmitted

Lenses and Optics 83

LENS

WALL

CAMERA

SIDE VIEW

PLAN TOP VIEW

LENS HORIZONTAL FOV

θH

θV

30 ft

30.6 ft

23.4 ft

FORMAT: 1/4 INCH

W = 30.6 ft

H = 23.4 ft

D = 30 ft

CAMERA

30 ft

30.6 ft

30 ft

FIGURE 4-9 Determining lobby lens horizontal and vertical FOVs

through the lens system varies inversely as the square of the lens f-number (K = constant):

I = K �f /#�2

Long-FL lenses are larger (and costlier) than short-FL lenses, due to the cost of the larger optical elements. It can be seen from Equation 4-11 that the larger the d is made, the smaller the f/# is, i.e. more light gets to the camera sensor. The more light the lens can collect and transfer to the camera image sensor the better the pic- ture quality: a larger lens permits the camera to operate at lower light levels. This light-gathering ability depends on the size (diameter) of the optics: the larger the optics the more the light that can be collected.

Most human eyes have the same size lens (approxi- mately 7 mm lens diameter). In video systems, however, the lens size (the diameter of the front lens) varies over a wide range. The optical speed of video lenses varies sig- nificantly: it varies as the square of the diameter of the lens. This means a lens having a diameter twice that of another will pass four times as much light through it. Like a garden hose, when the diameter is doubled, the flow is quadrupled (Figure 4-10).

The more the light passing through a lens and reaching the video sensor the better the contrast and picture image

quality. Lenses with low f-numbers, such as f/1.4 or f/1.6, pass more light than lenses with high f-numbers. The lens optical speed is related to the FL and diameter by the equation f/# = focal length/diameter. So the larger the FL given the same lens diameter, the “slower” the lens (less light reaches the sensor). A slow lens might have an f-number of f/4 or f/8.

Most lenses have an iris ring usually marked with num- bers such as 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, C, repre- senting optical speed, f-numbers, or f-stops. The difference between each of the iris settings represents a difference of a factor of 2 in the light transmitted by the lens. Opening the lens from, say, f/2.0 to f/1.4 doubles the light trans- mitted. Only half the light is transmitted when the iris opening is reduced from, say, f/5.6 to f/8. Changing the iris setting two f-numbers changes the light by a factor of 4 (or 1/4), and so on. Covering the f/# range from f/1.4 to f/22 spans a light-attenuation range of 256 to 1. The C designation on the lens indicates when the lens iris is closed and no light is transmitted.

In general, faster lenses collect more light energy from the scene, are larger, and are more expensive. In calculating the overall cost of a video camera lens system, a more expensive, fast lens often overrides the higher cost incurred if a more sensitive camera is needed or additional lighting must be installed.

84 CCTV Surveillance

WATER

LENS

D = 2

D = DIAMETER

LIGHTFLOW TRANSMISSIONANALOGY LENS

WATER FLOW: 4X

LIGHT TRANSMISSION: 4X

f / # = OPTICAL SPEED

f/2

f/4

HOSE

D = 1

FIGURE 4-10 Light transmission through a lens

4.2.7 Depth of Field

The depth of field in an optical system is the distance that an object in the scene can be moved toward or away from the lens and still be in good focus. In other words, it is the range of distance toward and away from the cam- era lens in which objects in the scene remain in focus. Ideally this range would be very large: say, from a few feet from the lens to hundreds of feet, so that essentially all objects of interest in the scene would be in sharp focus. In practice this is not achieved because the depth of field is: (1) inversely proportional to the focal length, and (2) directly proportional to the f-number. Medium to long FFL lenses operating at low f-numbers—say, f/1.2 to f/4.0—do not focus sharp images over their useful range of from 2 or 3 feet to hundreds of feet. Long focal length lenses—say, 50–300 mm—have a short depth of field and can produce sharp images only over short distances and must be refocused manually or automati- cally (auto-focus) when viewing objects at different scene distances.

When these lenses are used with their iris closed down to, say, f/8 to f/16, the depth of field increases significantly and objects are in sharp focus at almost all distances in the scene. Short focal length lenses (2�7–5 mm) have a long depth of field. They can produce sharp images from a few feet to 50–100 feet even when operating at low f-numbers

4.2.8 Manual and Automatic Iris

The lens iris is either manually or automatically adjusted to optimize the light level reaching the sensor (Figure 4-3). The manual iris is adjusted with a ring on the lens. The auto-iris uses an internal mechanism and motor (or gal- vanometer) to adjust the iris.

4.2.8.1 Manual Iris

The manual-iris video lens has movable metal “leaves” forming the iris. The amount of light entering the cam- era is determined by rotating an external iris ring, which opens and closes these internal leaves. Figure 4-11 shows a manual iris FFL lens and the change in light transmitted through it at different settings of the iris.

Solid-state CCD and CMOS camera sensors can oper- ate over wide light-level changes with manual-iris lenses but require automatic-iris lenses when used over their full light-level range, that is, from bright sunlight to low- level nighttime lighting. Some solid-state cameras use electronic shuttering (Section 5.5.3) and do not require an automatic-iris lens.

4.2.8.2 Automatic-Iris Operation

Automatic-iris lenses have an electro-optical mechanism whereby the amount of light passing through the lens is adjusted depending on the amount of light available from the scene and the sensitivity of the camera.

The camera video signal provides the information used for adjusting the light passing through the lens. The sys- tem works something like this: if a scene is too bright for the camera, the video signal will be strong (large in ampli- tude). This large signal will activate a motor or galvanome- ter that causes the lens iris circular opening to become smaller in diameter, thereby reducing the amount of light reaching the camera. When the amount of light reach- ing the camera produces a predetermined signal level, the motor or galvanometer in the lens stops and main- tains that light level through the lens. Likewise if too little light reaches the camera, the video camera signal level is small and the automatic-iris motor or galvanometer opens up the iris diaphragm, allowing more light to reach the camera. In both the high and the low light level con- ditions the automatic-iris mechanism produces the best

Lenses and Optics 85

MANUAL IRIS

LENS

1 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256 1/512 0

C = LENS IRIS FULLY CLOSED

LIGHT TRANSMISSION

1 1.4 2.0 2.8 4.0 5.6 8.0 11.0 22.0 C16.0

f/# MARKINGS

f/#

NOTE: REFERENCE LIGHT LEVEL = 1 AT f/1.4

EACH INCREASE OR DECREASE IN f/# REPRESENTS ONE f STOP

FOCUSING RING

FIGURE 4-11 Lens f/# vs. light transmission

contrast picture. Automatic-iris lenses are available to com- pensate the full range of light, from bright sunlight to darkness.

There are two types of automatic-iris lenses: direct drive and video drive. The two methods used to control these two lens types are: DC motor (or galvanometer) drive or video signal drive. With the DC drive method the camera has all the electronics and directly drives the DC motor in the lens with a positive or a negative signal to open and close the iris depending on the light level. With the video method the camera video signal drives the electron- ics in the lens which then drives the DC motor (or gal- vanometer) in the lens. Figure 4-12 shows some common automatic-iris lenses.

A feature available on some automatic-iris lenses is called average-peak response weighting which permits opti- mizing the picture still further based on the variation in lighting conditions within the scene. Scenes with high-contrast objects (bright headlight, etc.) are bet- ter compensated for by setting the automatic-iris con- trol to peak, so that the lens ignores (compensates for) the bright spots and highlights in the scene (see Section 5.5.5). Low-contrast scenes are better compen- sated by setting the control to average. Figure 4-13 illus- trates some actual scenes obtained when these adjust- ments are made. Automatic-iris lenses should only be used with cameras having a fixed video gain in their system.

Automatic-iris lenses are more expensive than their man- ual counterparts, with the price ratio varying by about two or three to one.

4.2.9 Auto-Focus Lens

Auto-focus lenses were originally developed for the con- sumer camcorder market and are now available to the security market (similar to the solid-state sensor evolu- tion). There are two types of auto-focus techniques in use. One auto-focus system uses a ranging (distance measur- ing) means to automatically focus the scene image onto the sensor. A second type analyzes the video signal in by means of DSP electronics, and forces the lens to focus on the target in the scene. The function of both these types of systems is to keep objects of interest in focus on the camera sensor even though they move toward or away from the lens. These lenses are particularly useful when a person (or vehicle) enters a camera FOV and moves toward or away from the camera. The auto-focus lens changes focus from the surrounding scene and focuses on the moving object (automatically) to keep the mov- ing object in focus. Various types of automatic-focusing techniques are used, including: (1) active IR ranging, (2) ultrasonic wave, (3) solid-state triangulation, and (4) video signal DSP.

86 CCTV Surveillance

(A) DC MOTOR IN LENS, ELECTRONICS IN CAMERA (B) VIDEO ELECTRONICS IN CAMERA

FIGURE 4-12 Automatic-iris fixed focal length (FFL) lenses

(A) HIGH CONTRAST SCENES OPTIMIZED USING MEDIUM RESPONSE WEIGHTING

(B) NORMAL CONTRAST SCENES OPTIMIZED USING PEAK RESPONSE WEIGHTING

FIGURE 4-13 Automatic-iris enhanced video scenes

4.2.10 Stabilized Lens

A stabilized lens is used when it is necessary to remove unwanted motion of the lens and camera with respect to the scene being viewed. Applications for stabilized lenses include handheld cameras, cameras on moving ground vehicles, airborne platforms, ships, and cameras on towers

and buildings. Stabilized lenses can remove significant image vibration (blurring) in pan/tilt mounted cameras that are buffeted by wind or the motion caused by the mov- ing vehicle. The stabilized lens system has movable optical components and/or active electronics that compensate for (move in the opposite direction to) the relative motion between the camera and the scene. An extreme example

Lenses and Optics 87

of a stabilized video camera system is the video image from a helicopter. The motion compensation results in a steady, vibration free scene image on the monitor. Figure 4-14 shows a stabilized security lens and samples of pictures taken with and without the stabilization on.

4.3 FIXED FOCAL LENGTH LENS

Video lenses come in many varieties, from the simple, small, and inexpensive mini pinhole and “bottle cap” lenses to complex, large, expensive, motorized, automatic- iris zoom lenses. Each camera application requires a spe- cific scene to be viewed and a specific intelligence to be extracted from the scene if it is to be useful in a secu- rity application. Monitoring a small front lobby or room to see if a person is present may require only a simple lens. It is difficult, however, to determine the activity of a person 100–200 feet away in a large showroom. Appre- hending a thief or thwarting a terrorist may require a high-quality, long FL zoom lens and a high resolution camera mounted on a pan/tilt platform. Covert cameras using pinhole lenses are often used to uncover internal theft, shoplifting, or other inappropriate or criminal activ- ity. They can be concealed in inconspicuous locations, installed quickly, and moved on short notice. The follow- ing sections describe common and special lens types used in video surveillance applications.

The majority of lenses used in video applications are FFL lenses. Most of these lenses are available with a man- ual focusing ring to adjust the amount of light passing through the lens and reaching the image sensor. The very short FL lenses (less than 6 mm) often have no manual iris. FFL lenses are the workhorses of the industry. Their attributes include low cost, ease of operation, and long life. Most FFL lenses are optically fast and range in speed from f/1.2 to f/1.8, providing sufficient light for most cameras to produce an excellent quality picture. The manual-iris lenses are suitable for medium light-level when used with most solid-state cameras.

Most FFL lenses have a mount which in the industry for attaching the lens to the camera is called a C or CS mount (Figure 4-15).

The C mount has been a standard in the CCTV industry for many years while the CS mount was introduced in the mid-1990s to match the trend toward smaller camera sen- sor formats and their correspondingly smaller lens require- ments. The C and CS mount has a 1 inch 32 threads per inch thread. Most security surveillance cameras are now manufactured with a CS mount and supplied with a 5 mm thick spacer adapter ring which allows a C mount lens to be attached to the CS mount camera. The C mount focuses the scene image 0.69 inches (17.5 mm) behind the lens onto the camera sensor. The CS mount focuses the scene image 0.394 inches (10 mm) behind the lens onto the sensor. Commonly used CS mount FLs vary from 2.5 mm

(A) LENS

(D) STABILIZED IMAGE

(B) LENS

FIGURE 4-14 Stabilized lenses and results

88 CCTV Surveillance

1"-32 TPI

CCD/CMOS SENSOR

17.526 mm (0.69")

12.5 mm (0.492")

BACK FOCAL DISTANCE

IMAGE PLANE

MECHANICAL BACK FOCAL DISTANCE

CCD/CMOS

SENSOR

1"-32 TPI

C MOUNT CS MOUNT MECHANICAL LENS GEOMETRY

FLANGE BACK FOCAL LENGTH (F,f )

SCENE

TPI = THREADS PER INCH

FIGURE 4-15 C and CS mount lens mounting dimensions

(wide-angle) to 200 mm (telephoto). The C mount is also used for long FL lenses having large physical dimensions. Large optics are designed to be used with almost any sen- sor format size from 1/4 to 1 inch. Lenses with FLs longer than approximately 300 mm are large and expensive. As the FL becomes longer the diameter of the lens increases and costs escalate accordingly. Most FFL lenses are avail- able in a motorized or automatic-iris version. These are necessary when they are used with LLL ICCD cameras in daytime and nighttime applications where light level must be controlled via an automatic iris or neutral density filters depending on the scene illumination.

With the widespread use of smaller cameras and lenses a new set of lens–camera mounts developed. They were not given any special name but are referred to as 11 mm, 12 mm (the most common), and 13 mm mounts. The dimensions refer to the diameter of the thread on the lens and the camera mount. Figure 4-16 shows these lens mounts. Note that the threads are not all the same (the 13 mm mount is different from the 11 mm and 12 mm).

4.3.1 Wide-Angle Viewing

While the human eye has peripheral vision and can detect the presence and movement of objects over a wide angle

(160�), the eye sees a focused image in only about the central 10� of its FOV. No video camera has this unique eye characteristic, but a video system’s FOV can be increased (or decreased) by replacing the lens with one having a shorter (or longer) FL. The eye cannot change its FOV without the use of external optics.

Choosing different FL lenses brings trade-offs: reducing the FL increases the FOV but reduces the magnification, thereby making objects in the scene smaller and less dis- cernible (i.e. decreasing resolution). Increasing the FL has the opposite effect.

To increase the FOV of a CCTV camera, a short-FL lens is used. The FOV obtained with wide-angle lenses can be calculated from Equations 4-1, 4-2, 4-3, and 4-4, or by using Table 4-4, or the Lens Finder Kit. For example, substituting an 8 mm FL, wide-angle lens for a 16 mm lens on any camera doubles the FOV. The magnification is reduced to one-half, and the camera sees “twice as much but half as well.” By substituting a 4 mm FL lens for the 16 mm lens, the FOV quadruples. We see sixteen times as much scene area but one-fourth as well.

A 2.8 mm FL lens is an example of a wide-angle lens; it has an 82� horizontal by 67� vertical FOV on a 1/3-inch sensor. A super wide FOV lens for a 1/2-inch sensor is the 3.5 mm FL lens, with an FOV approximately 90� hor- izontal by 75� vertical. Using a wide-angle lens reduces

THREAD: 13 mm DIA. × 1.0 mm PITCH

SENSOR

MOST COMMON MOUNT: 12 mm DIA. × 0.5 mm PITCH

THREAD: 12 mm DIA. × 0.5 mm PITCH THREAD: 10 mm DIA. x 0.5 mm PITCH

BACK FOCAL LENGTH RANGES

FROM 3 mm TO 9 mm

FIGURE 4-16 Mini-lens mounting dimensions

Lenses and Optics 89

FL = 4.0 mm FL = 25 mm FL = 75 mm NARROW ANGLE (TELEPHOTO)

NOTE: ANGULAR FOV SHOWN FOR 1/3-, 1/2-, AND 2/3-INCH FORMAT SENSORS

1/2"

1/3"

TELEPHOTO (SHADED)

NORMAL FOR 1" SENSOR

FL = 8 MM NORMAL FOR 1/3" SENSOR

WIDE ANGLE

1/4"

FIGURE 4-17 Wide-angle, normal, and narrow-angle (Telephoto) FFL lenses vs. format

the resolution or ability to discern objects in a scene. Figure 4-17 shows a comparison of the FOV seen on 1/4-, 1/3-, and 1/2-inch format cameras with wide-angle, nor- mal, and telephoto lenses.

4.3.2 Narrow-Angle Telephoto Viewing

When the lens FL increases above the standard M = 1 magnification condition the FOV decreases and the mag- nification increases. Such a lens is called a medium- or narrow-angle (telephoto) lens. The lens magnification is determined by Equations 4-5, 4-10, 4-8, and 4-9 for the reference (1 inch) and three commonly used sensor sizes (see also Table 4-4 and the Lens Finder Kit).

Outdoor security applications often require viewing scenes hundreds and sometimes thousands of feet away from the camera. To detect and/or identify objects, persons, or activity at these ranges requires very long- FL lenses. Long-FL lenses between 150 and 1200 mm are usually used outdoors to view these parking lots or other remote areas. These large lenses require very stable mounts and rugged pan-tilt drives to obtain good picture quality. The lenses must be large (3–8 inches in diameter)

to collect enough light from the distant scene and have usable f-numbers (f/2.5 to f/8) for the video camera to produce a good picture on the monitor.

Fixed focal length lenses having FLs from 2.6 mm up to several hundred millimeters are refractive- or glass-type. Above approximately 300 mm FL, refractive glass lenses become too large and expensive, and reflective mirror optics or mirror and glass optics are used to achieve opti- cally fast (low f-number) lenses with lower weight and size. These long FL telephoto lenses, called “Cassegrain” or “catadioptric lenses,” cost hundreds to thousands of dollars. Figure 4-18 shows a schematic of these lenses, a 700 mm f/8.0 and a 300 mm f/5.6 lens used for long- range outdoor surveillance applications.

4.4 VARI-FOCAL LENS

The vari-focal lens is a variable focal length lens developed to be used in place of an FFL lens (Figure 4-19).

In general it is smaller and costs much less than a zoom lens. The advantage of the vari-focal lens over an FFL lens is that its focal length and FOV can be changed manually

90 CCTV Surveillance

SCENE

WINDOW

L = CORRECTING LENSES

SENSOR

FOCAL PLANE

M1 = PRIMARY MIRROR M2 = SECONDARY MIRROR

LIGHT FROM DISTANT

FIGURE 4-18 Long-range, long-focal length catadioptric lenses

SENSOR VARI-FOCAL LENS

FOCAL LENGTH 3–8 mm

3 mm FOV

8 mm FOV

FIGURE 4-19 Vari-focal lens configuration

by rotating the barrel on the lens. This feature makes it convenient to adjust the lens FOV to a precise angle while installed on the camera. The lenses were developed to be used in place of FFL lenses to “fine tune” the FL for a particular application. Having the ability to adjust the FL “on the job” makes it easier for the installer and at the same time permits the customer to select the exact FOV necessary to observe the desired scene area. One minor inconvenience of the vari-focal lens is that it must

be refocused each time the FL is changed. Typical vari- focal lenses are available with focal lengths of: 3–8 mm, 5–12 mm, 8–50 mm, 10–120 mm (Table 4-6). With just these few lenses focal lengths of from 1.8–120 mm and 144–1�6� FOVs can be covered continuously (i.e.—any focal length in the range). The vari-focal lenses are a sub- set and simplified version of zoom lenses but they are not a suitable replacement for the zoom lens in a variable FOV pan/tilt application.

Lenses and Optics 91

HORIZONTAL ANGULAR FOV (DEG.) 1/4 INCH 1/3 INCH 1/2 INCH 2/3 INCH

FOCAL LENGTH

(mm)

ZOOM RATIO

FORMAT (INCH)

OPTICAL SPEED

f/# WIDE TELE WIDE TELE WIDE TELE WIDE TELE 1.4–3.1 2.2:1 1/3 1.4 121 69.5 185 94.5 1.6–3.4 2.1:1 1/3 1.4 135 101 180 84.3 1.8–3.6 2:1 1/3 1.6 144 79.0 2.2–6 2.7:1 1/4 1.2 90.0 34.7 2.7–12 4.4:1 1/3 1.2 75 56 97.4 23.8 2.8–12 4.3:1 1/3 1.4 73 54.7 97.4 24.1 3.0–8 2.7:1 1/3 1.4 67.0 26.0 89.5 34.0 3.0–8 2.7:1 1/3 1.0 67.0 27 91 36 3.5–8 2.3:1 1/3 1.4 58.7 26.5 79.8 35.4 3.6–18 5:1 1/2 1.8 54.4 11.5 72.3 54.3 95.9 20.0 4.5–12.5 2.8:1 1/2 1.2 45.9 34.4 61.2 45.9 81.6 30.0 5.0–50 10:1 1/3 1.3 39.0 4.2 52 5.6 5.5–82.5 15:1 1/3 1.8 35.3 2.5 47.1 3.3

6–12 2:1 1/2 1.4 31.6 16.8 42.1 22.4 56.1 29.9 6–15 2.5:1 1/2 1.4 33.1 14.4 44.1 19.2 59.1 25.7 6–60 10:1 1/3 1.6 32.7 3.5 43.6 4.7 7–70 10:1 1/2 1.8 29.0 3.0 38.2 4.0 50.0 5.1 8–16 2:1 1/2 1.6 24.5 12.6 33.6 16.8 43.5 22.4 59.8 30.8 8–80 10:1 1/2 1.6 25 26 33.0 3.5 42.9 4.6

10–30 3:1 1/2 1.4 20 7.1 27 9.4 36 12.5 10–40 4:1 1/3 1.4 20.6 5.3 27.5 7.0 20–100 5:1 1/3 1.6 10.2 2.1 13.6 2.8

NOTE: HORIZONTAL ANGULAR FOV FROM MANUFACTURERS’ SPECIFICATIONS

Table 4-6 Representative Vari-Focal Lenses—Focal Length, Vari-Focal Zoom Ratio vs. Sensor Format, Horizontal FOV

4.5 ZOOM LENS

Zoom and vari-focal lenses are variable FL lenses. The lens components in these assemblies are moved to change their relative physical positions, thereby varying the FL and angle of view through a specified range of magnifications. Prior to the invention of zoom optics, quick conversion to different FLs was achieved by mounting three or four different FFL lenses on a turret with a common lens mount in front of the CCTV camera sensor and rotating each lens into position, one at a time, in front of the sensor. The lenses usually had wide, medium, and short FLs to achieve different angular coverage. This turret lens was obviously not a variable-FL lens and had limited use.

4.5.1 Zooming

Zooming is a lens feature that permits seeing detailed close-up views (high magnification) of a subject (scene target) or a broad (low magnification), overall view of an area. Zoom lenses allow a smooth, continuous change in the angular FOV. The angle of view can be made narrower or wider depending on the zoom setting. As a result, a scene can be made to appear close-up (high magnification) or far away (low magnification), giving the impression of camera movement toward or away from the

scene, even though the camera remains in a fixed posi- tion. Figure 4-20 shows the continuously variable nature of the zoom lens and how the FOV of the video camera can be changed without replacing the lens.

To implement zooming, several elements in the lens are physically moved to vary the FL and thereby vary the angular FOV and magnification. Tables 4-1, 4-2, 4-3, and 4-4, and the Lens Finder Kit can be used to determine the FOV for any zoom lens. By adjusting the zoom ring setting, one can view narrow-, medium-, or wide-angle scenes. This allows a person to view a scene with a wide-angle perspec- tive and then close in on one portion of the scene that is of specific interest. The zoom lens can be made significantly more useful and providing the camera a still wider FOV by mounting it on a pan/tilt platform controlled from a remote console. The pan/tilt positioning and the zoom lens variable FOV from wide to narrow angle and anywhere in between provide a large dynamic FOV capability.

4.5.2 Lens Operation

The zoom lens is a cleverly designed assembly of lens elements that can be moved to change the FL from a wide angle to a narrow angle (telephoto) while the image on the sensor remains in focus (Figure 4-21). This is a significant difference from the vari-focal lens

92 CCTV Surveillance

CAMERAWIDE-ANGLE LIMIT

NARROW-ANGLE TELEPHOTO LIMIT

HORIZONTAL FIELD OF VIEW

VERTICAL FIELD OF VIEW

ZOOM LENS

FIGURE 4-20 Zoom lens variable focal length function

FOCUS RING

ZOOM RING

IRIS RING

FRONT

OBJECTIVE GROUP

MOVABLE ZOOM

GROUP

REAR STATIONARY

GROUP

IRIS

FOCUSING

RELAY

CAMERA SENSOR

FIGURE 4-21 Zoom lens configuration

Lenses and Optics 93

which must be re-focused each time its FL is changed (Section 4.4).

A zoom FL lens combines at least three moveable groups of elements:

1. The front focusing objective group that can be adjusted over a limited distance with an external focus ring to initially fine-focus the image onto the camera sensor.

2. A movable zoom group located between the front and the rear group that moves appreciably (front to back) using a separate external zoom ring. The zoom group also contains corrective elements to optimize the image over the full zoom range. Other lenses are also moved a small amount to automatically adjust and keep the image on the sensor in sharp focus, thereby eliminating subsequent external adjustment of the front focusing group.

3. The rear stationary relay group at the camera end of the zoom lens that determines the final image size when it is focused on the camera sensor.

Each lens group normally consists of several elements. When the zoom group is positioned correctly, it sees the image produced by the objective group and creates a new image from it. The rear relay group picks up the image from the zoom group and relays it to the cam- era sensor. In a well-designed zoom lens a scene in focus at the wide-angle (short-FL) setting remains in focus at the narrow-angle (telephoto) setting and everywhere in between.

4.5.3 Optical Speed

Since the FL of a zoom lens is variable and its entrance aperture is fixed, its f-number is not fixed (see Equa- tion 4-11). For this reason, zoom lens manufacturers often list the f-number for the zoom lens at the wide and nar- row FLs, with the f-number at the wide-angle setting being

faster (more light throughput, lower f-number) than at the telephoto setting. For example, a 11–110 mm zoom lens may be listed as f/1.8 when set at 11 mm FL and f/4 when set at 110 mm FL. The f-number for any other FL in between the two settings lies in between these two values.

4.5.4 Configurations

Many manufacturers produce a large variety of manual and motorized zoom lenses suitable for a wide variety of applications. Figure 4-22 shows two very different zoom lenses used for surveillance applications.

The manual zoom lens shown has a 8�5–51 mm FL (6:1 zoom ratio) and has an optical speed of f/1.6. The long range lens shown has a large zoom ratio of 21:1. This lens has an FL range of 30–750 mm and speed of f/4.6.

Figure 4-23 shows the FOVs obtained from a 11–110 mm FL zoom lens on a 1/2-inch sensor camera at three zoom FL settings.

Table 4-7 is a representative list of manual and motor- ized zoom lenses, from a small, lightweight, inexpensive 8–48 mm FL zoom lens to a large, expensive, 13.5–600 mm zoom lens used in high-risk security areas by industry, mil- itary, and government agencies.

Zoom lenses are available with magnification ratios from 6:1 to 50:1. Many have special features, including remotely controlled preset zoom and focus positions, auto-focus and stabilization.

4.5.5 Manual or Motorized

The FL of a zoom lens is changed by moving an exter- nal zoom ring either manually or with an electric motor. When the zoom lens iris, focus, or zoom setting must be adjusted remotely, a motorized lens with a remote con- troller is used. The operator can control and change these

(A) MANUAL (B) MOTORIZED

FIGURE 4-22 Manual and motorized zoom lenses

94 CCTV Surveillance

SCENE

ZOOM LENS 11–110 mm NOTE: THE LENS HAS A CIRCULAR FOV

BUT THE SENSOR 4:3 ASPECT RATIO FOV IS SHOWN

11 mm FL

24 mm FL

110 mm FLWIDE: 46° MEDIUM: 24° NARROW: 5°

46°

24°

5°

2 /3-INCH SENSOR FORMAT

HORIZONTAL ZOOM ANGLES

FIGURE 4-23 Zoom lens FOV at different focal length settings

HORIZONTAL ANGULAR FOV (DEG.) 1/4 INCH 1/3 INCH 1/2 INCH 2/3 INCH

FOCAL LENGTH

(mm)

ZOOM RATIO

FORMAT (INCH)

OPTICAL SPEED:

f/# WIDE T ELE WIDE TELE WIDE TELE WIDE TELE

4.5–54 12:1 1/4 1.1 43.5 3.7 4.6–28 6:1 1/4 1.0 41.6 7.5 5.5–77 14:1 1/3 1.4 36.2 2.6 47.1 3.5 5.5–187 34:1 1/3 1.8 35.0 1.1 46.6 1.5 5.7–34.2 6:1 1/3 1.0 34.5 6.1 46 8.1 5.8–121.8 21:1 1/3 1.6 33.8 1.7 45 2.3

6–72 12:1 1/3 1.5 33.4 2.8 43.6 3.8 6–90 15:1 1/3 1.2 33.0 2.3 43.8 3.1

7.5–105 14:1 1/2 1.4 27.0 2.0 35.5 2.6 46.2 3.5 8–48 6:1 1/2 1.0 24.9 4.4 33.0 5.9 43.2 7.7 8–96 12:1 1/2 2.0 25.3 2.2 33.4 2.8 43.5 3.7 8–160 20:1 1/2 2.0 25.1 1.3 33.4 1.7 43.6 2.3 9–180 20:1 1/3 1.2 22.6 1.2 30.3 1.5

10.5–105 10:1 2/3 1.4 18.6 2.0 24.8 2.6 33.0 3.5 45.5 4.8 10–140 14:1 2/3 1.9 19.9 1.5 26.4 2.0 35.0 2.7 47.5 3.6 10–200 20:1 1/2 2.5 20.3 1.1 27.0 1.4 35.5 1.8 10–300 30:1 1/2 1.5 20.0 0.7 26.6 0.9 35.5 1.25

12.5–75 6:1 2/3 1.6 16.1 2.8 21.4 3.7 28.4 4.9 38.8 6.7 12–120 10:1 1/2 1.8 16.6 1.7 22.1 2.3 29.4 3.1 12–240 20:1 1/2 1.6 17.2 0.9 23.0 1.2 30.8 1.6 16–160 10:1 1 1.8 16.8 1.8 22.4 2.4 30.8 3.2 44.9 4.6 10–500 50:1 1/2 4.0 13.7 0.3 18.2 0.4 35.5 0.7

NOTE: NOMINAL HORIZONTAL ANGULAR FOV FROM MANUFACTURERS’ SPECIFICATION

Table 4-7 Representative Motorized Zoom Lenses—Focal Length, Zoom Ratio vs. Sensor Formats, Horizontal FOV

Lenses and Optics 95

settings remotely using toggle switch controls on the con- sole or automatically through preprogrammed software. The motor and gear mechanisms effecting these changes are mounted within the zoom lens. Manual zoom lenses are not very practical for surveillance since an operator is not located at the camera location and cannot manually adjust the zoom lens.

4.5.6 Adding a Pan/Tilt Mechanism

A zoom lens and camera pointed in a fixed direction pro- vides limited viewing. When the zoom lens is viewing wide angle it sees a large FOV, but when it is zoomed to a nar- row angle it will zoom in and magnify only in one pointing direction—straight ahead of the camera. This is of limited use unless it is pointing at a area of importance such as an entrance door, entry/exit gate, receptionist, i.e. one single location.

To fully utilize a zoom lens it is mounted on a pan-tilt mechanism so that the lens can be pointed in almost any direction (Figure 4-24).

By varying the lens zoom control and moving the pan/tilt platform, a wide dynamic FOV is achieved. The pan-tilt and lens controller remotely adjusts pan, tilt, zoom, and focus. The lens usually has an automatic iris,

but in some cases the operator can choose a manual- or automatic-iris setting on the lens or the controller. In surveillance applications, one shortcoming of a pan-tilt mounted zoom lens is the existence of “dead zone” viewing areas since the lens cannot point and see in all directions at once.

4.5.7 Preset Zoom and Focus

In a computer-controlled surveillance system a motorized zoom lens with electronic preset functions is used. In this mode of operation as a preset zoom lens, the zoom and focus ring positions are monitored electrically and memo- rized by the computer during system setup. These settings (presets) are then automatically repeated on command by the computer software at a later time. In this surveillance application, this feature allows the computer to point the camera–lens combination according to a set of predeter- mined conditions and adjust pointing and the zoom lens FL and focus: i.e. (1) azimuth and elevation angle, (2) focused at a specific distance and (3) iris set to a spe- cific f-number opening. When a camera needs to turn to another preset set of conditions in response to an alarm sensor or other input, the preset feature eliminates the need for human response and significantly reduces the time to acquire a new target.

SCENE

PAN/ TILT POINTING RANGE

ZOOM LENS NARROW FOV

ZOOM LENS WIDE

FOV

CAMERA LENS PAN/ TILT PLATFORM

RANGE POINTING PAN/ TILT

ZOOM LENS CAN BE POINTED ANYWHERE WITHIN PAN/ TILT POINTING RANGE

FIGURE 4-24 Dynamic FOV of pan/tilt-mounted zoom lens

96 CCTV Surveillance

4.5.8 Electrical Connections

The motorized zoom lens contains electronics, motors, and clutches to control the movement of the zoom, focus, and iris adjustment rings, and end-of-travel limit switches to protect the gear mechanism.

Since the electrical connections have not been standard- ized among manufacturers, the manufacturer’s lens wiring diagram must be consulted for proper wiring. Figure 4-25 shows a typical wiring schematic for zoom, focus, and iris mechanisms. The zoom, focus, and iris motors are con- trolled with positive and negative DC voltages from the lens controller, using the polarity specified by the manu- facturer.

4.5.9 Initial Lens Focusing

To achieve the performance characteristics designed into a zoom lens, the lens must be properly focused onto the camera sensor during the initial installation. Since the lens operates over a wide range of focal lengths it must be tested and checked to ensure that it is in focus at the wide-angle and telephoto settings. To perform a crit- ical focusing of the zoom lens the aperture (iris) must

be wide open (set to the lowest f-number) for all back- focus adjustments of the camera sensor. This provides the conditions for a minimum depth of field, and the con- ditions to perform the most critical focusing. Therefore, adjustments must be performed in subdued lighting, or with optical filters in front of the lens, to reduce the light and allow the lens iris to open fully to get minimum depth of field. The following steps should be followed to focus the lens:

1. With the camera operating, view an object at least 50 feet away.

2. Make sure the lens iris is wide open so that focusing is most critical.

3. Set the lens focus control to the extreme far position. 4. Adjust the lens zoom control to the extreme wide-angle

position (shortest FL). 5. Adjust the camera sensor position adjustment control

to obtain the best focus on the monitor. 6. Move the lens zoom to the extreme telephoto (longest

FL) setting. 7. Adjust the lens focus control (on the controller) for the

best picture. 8. Re-check the focus at the wide-angle (position of short-

est FL).

CAMERA

IRIS

ZOOM

ZOOM LENS

FOCUS

AUTO

MANUAL

NEAR

FAR

TELE

WIDE

IRIS

FOCUS CONTROL

ZOOM CONTROL

CONTROL CONSOLE CONNECTOR

SUPPLY VOLTAGE *

VIDEO SIGNAL

FOCUS + 12 V NEAR ***

ZOOM + 12 V TELE *** COMMON

REQUIRES POSITIVE AND NEGATIVE VOLTAGE

REMOTE CONTROL **

* DEPENDS ON MANUFACTURER: RANGES FROM 8 TO 12 VDC TYPICAL

** REQUIRES POSITIVE AND NEGATIVE VOLTAGE: ±6 VDC MAX TYPICAL

*** DEPENDING ON THE CONTROLLER THIS MAY BE ±6 OR ±12 VDC MAX

FIGURE 4-25 Motorized zoom lens electrical configuration

Lenses and Optics 97

4.5.10 Zoom Pinhole Lens

Pinhole lenses with a small front lens element are com- mon place in covert video surveillance applications. Zoom pinhole lenses while not as common as FFL pinhole lenses are available in straight and right-angle configuration. One lens has an FL range of 4−12 mm and an optical speed of f/4.0.

4.5.11 Zoom Lens–Camera Module

The requirement for a compact zoom lens and camera combination has been satisfied with the zoom lens–camera module. This module evolved out of a requirement for a lightweight, low-inertia camera lens for use in high-speed pan-tilt dome installations in casinos and retail stores. The camera–lens module has a mechanical cube configuration (Figure 4-26) so that it can easily be incorporated into small pan-tilt dome housings and be pointed in any direc- tion at high speeds.

The module assembly includes the following compo- nents and features: (1) rugged, compact mechanical structure suitable for high-speed pan-tilt platforms, (2) large optical zoom ratio, typically 20:1, and (3) sensitive 1/4- or 1/3-inch solid-state color camera with excellent sensitivity and resolution. Options include: (1) automatic- focus capability, (2) image stabilization, and (3) elec- tronic zoom.

4.5.12 Zoom Lens Checklist

The following should be considered when applying a zoom lens:

FIGURE 4-26 Compact zoom lens–camera cube

• What FOV is required? See Tables 4-1, 4-2, 4-3, 4-4, and the Lens Finder Kit.

• Can a zoom lens cover the FOV or must a pan-tilt plat- form be used?

• Is the scene lighting constant or widely varying? Is a manual or automatic iris required?

• What is the camera format: 1/4-, 1/3-, 1/2-inch? • What is the camera lens mount type: C, or CS? • Is auto-focus or stabilization needed? • Is electronic zoom required to extend the FL range?

Zoom lenses on pan-tilt platforms significantly increase the viewing capability of a video system by providing a large range of FLs all in one lens. The increased complexity and precision required in the manufacture of zoom lenses makes them cost three to ten times as much as an FFL lens.

4.6 PINHOLE LENS

A pinhole lens is a special security lens with a relatively small front diameter so that it can be hidden in a wall, ceiling, or some object. Covert pinhole lens–camera assem- blies have been installed in emergency lights, exit signs, ceiling-mounted lights, table lamps, and even disguised as a building sprinkler head fixture. Any object that can house the camera and pinhole lens and can disguise or hide the front lens element is a candidate for a covert installation. In practice the front lens is considerably larger than a pinhole, usually 0�06−0�38 inch in diameter, but nevertheless it can be successfully hidden from view. Vari- ations of the pinhole lens include straight or right-angle, manual or automatic iris, narrow-taper or stubby-front shape (Figure 4-27). The lenses shown are for use with C or CS mount cameras. Whether to use the straight or right-angle pinhole lens depends on the application. A detailed description and review of covert camera and pin- hole lenses are presented in Chapter 18.

4.6.1 Generic Pinhole Types

A feature that distinguishes two generic pinhole lens designs from each other is the shape and size of the front taper (Figure 4-28). The slow tapering design permits eas- ier installation than the fast taper and also has a faster optical speed, since the larger front lens collects more light.

The optical speed (f-number) of the pinhole lens is important for the successful implementation of a covert camera system. The lower the f-number of the lens, the more the light reaching the camera and the better the video picture. An f/2.2 lens transmits 2.5 times more light than an f/3.5. The best theoretical f-number is equal to

98 CCTV Surveillance

(A) MANUAL IRIS FAST TAPER (B) AUTOMATIC IRIS FAST TAPER

(E) RIGHT ANGLE AUTOMATIC IRIS SLOW TAPER

FIGURE 4-27 Straight and right-angle pinhole lenses

the FL divided by the entrance lens diameter (d ). From Equation 4-11:

f/# = FL d

For a pinhole lens, the light getting through the lens to the camera sensor is limited primarily by the diameter of the front lens or the mechanical opening through which it views. For this reason, the larger the lens entrance diam- eter, the more light gets through to the image sensor, resulting in a better picture quality, if all other conditions remain the same.

4.6.2 Sprinkler Head Pinhole

There are many types of covert lenses available for the security industry: pinhole, mini, fiber optic, camera-lenses covertly concealed in objects. The sprinkler head cam- era is a unique pinhole lens hidden in a ceiling sprin- kler fixture which makes it extremely difficult for an observer standing at floor level to detect or identify. This unique device provides an extremely useful covert surveil- lance system. Figure 4-29 shows the configuration and two versions of the sprinkler head lens, the straight and right-angle.

This pinhole lens and camera combination is con- cealed in and above a ceiling using a modified sprinkler head to view the room below the ceiling. For investiga- tive purposes, fixed pinhole lenses pointing in one spe- cific direction are usually suitable. To look in different directions there is a panning sprinkler head version. An integral camera-lens-sprinkler head design is shown in Section 18.3.5.

4.6.3 Mini-Pinhole

Another generic family of covert lenses is the mini-lens group (Figure 4-30). They are available in conventional on- axis and special off-axis versions. Their front mechanical shape can be flat or cone shaped.

These lenses are very small, some with a cone-shaped front, typically less than 1/2 inch diameter by 1/2 inch long and mount directly onto a small video camera. The front lens in these mini-lenses ranges from 1/16 inch to 3/8 inch diameter. The cone-shaped mini-lens is easier to install in many applications. These mini-lenses are opti- cally fast having speeds of f/1.4 to f/1.8 and can be used in places unsuitable for larger pinhole lenses. An f/1.4 mini- pinhole lens transmits five times more light than an f/3.5 pinhole lens. Mini-pinhole lenses are available in FLs from 2.1 to 11 mm and when combined with a good camera

Lenses and Optics 99

SLOW-TAPER BARREL FAST-TAPER BARREL

30° 55°

SMALL DIAMETER

LARGE DIAMETER

CEILING TILE

FIGURE 4-28 Short vs. long tapered pinhole lenses

CAMERA

SPRINKLER HEAD

MOVEABLE MIRROR

CEILING TILE

LENS/CAMERA FIELD OF VIEW

MANUAL IRIS

CEILING TILE

SPRINKLER HEAD

CAMERA

STRAIGHT RIGHT-ANGLE

RIGHT-ANGLE PINHOLE

SPRINKLER LENS

STRAIGHT PINHOLE

SPRINKLER LENS

FIGURE 4-29 Sprinkler head pinhole assembly installation

100 CCTV Surveillance

FIGURE 4-30 Mini-pinhole lenses

11 mm

8 mm IN C MOUNT

8 mm IN WATEC OFFSET MOUNT

11 mm IN WATEC OFFSET MOUNT

2.1 mm

3.8 mm

3.7 mm PINHOLE

8 mm

result in the fastest covert cameras available. A useful varia- tion of the standard mini-lens is the off-axis mini-lens. This lens is mounted offset from the camera axis, which causes the camera to look off to one side, up, or down, depend- ing on the offset direction chosen. Chapter 18 describes pinhole and mini-lenses in detail.

4.7 SPECIAL LENSES

There are several special video security lenses and lens func- tions that deserve consideration. These include: (1) a new panoramic 360� lens, (2) fiber-optic and bore scope, (3) split-image, (4) right-angle, (5) relay, (6) automatic-focus, (7) stabilized, and (8) long-range. The new panoramic lens must be integral with a camera and used with computer hardware and software (Section 5.10). The other special lenses are used in applications when standard FFL, vari- focal or zoom lenses are not suitable. The auto-focus and stabilizing functions are used to enhance the performance of zoom lenses, vari-focal, fixed focus lenses, etc.

4.7.1 Panoramic Lens—360�

There has always been a need to see “all around” i.e. an entire room or other location, seeing 360� with one panoramic camera and lens. To date, a 360� FOV cam- era system has only been achieved with multiple cameras and lenses and combining the images on a split-screen monitor. This lens is usually mounted in the ceiling of a room or on a tower. Panoramic lenses have been avail- able for many years but have only recently been combined with powerful digital electronics, sophisticated mathemat- ical transformations and compression algorithms to take

advantage of their capabilities. The availability of high res- olution solid-state cameras has made it possible to map a 360� by 90� hemispherical FOV into a standard rectan- gular monitor format with good resolution. Figure 4-31 shows two panoramic lens having a 360� horizontal FOV and a 90� vertical FOV.

In operation the lens collects light from the 360�

panoramic scene and focuses it onto the camera sensor as a donut-shaped image (Figure 4-32). The electronics and mathematical algorithm convert this donut-shaped panoramic image into the rectangular (horizontal and ver- tical) format for normal monitor viewing. (Section 2.6.5 describes the panoramic camera in detail.)

4.7.2 Fiber-Optic and Bore Scope Optics

Coherent fiber-optic bundle lenses can sometimes solve difficult video security applications. Not to be confused with the single or multiple strands of fiber commonly used to transmit the video signal over long distances, the coher- ent fiber-optic lens has many thousands of individual glass fibers positioned adjacent to each other. These thousands of fibers transmit a coherent image from an objective lens, over a distance of several inches to several feet, where the image is then transferred again by means of a relay lens to the camera sensor. A high-resolution 450 TV lines coher- ent fiber bundle consists of several hundred thousand glass fibers that transfer a focused image from one end of the fiber bundle to the other. Coherent optics means that each point in the image on the front end of the fiber bundle corresponds to a point at the rear end. Since the picture quality obtained with fiber-optic lenses is not as good as that obtained with all glass lenses, such lenses should only

Lenses and Optics 101

LENS SEES FULL HEMISPHERE:

360° × 180°

360°

90°

HORIZ. FOV

VERT. FOV

360° LENS

RAW DONUT IMAGE FROM 360° LENS

FIGURE 4-31 Panoramic 360� lens camera module

A B

LENS SEES FULL HEMISPHERE: 360° × 180°

IMAGE FORMED ON SENSOR

DONUT RING IMAGE REPRESENTING HEMISPHERE FOV: 360 H × 180 V

ALGORITHM TO CONVERT DONUT SHAPED PANORAMIC

IMAGE TO NORMAL LINEAR X,Y

LENS FEATURES:

SEES ENTIRE AREA INCLUDING DIRECTLY BELOW INFINITE DEPTH OF FIELD: NO FOCUSING REQUIRED NO MOVING PARTS FOR P/ T/ Z–ALL ELECTRONIC FLUSH CEILING (OR WALL) MOUNTING

PC SOFTWARE / HARDWARE

0°

90°

0° 360°

NORMAL 360° FORMAT IMAGE

SENSOR

DISPLAY NORMAL

360°

90°

HORIZ. FOV

VERT. FOV

360° PANORAMIC CAMERA /SENSOR

• • • •

FIGURE 4-32 Panoramic lens layout and description

102 CCTV Surveillance

(A) OBJECTIVE LENS: 8 mm OR 11 mm FL FIBER TYPE: RIGID CONDUIT RELAY LENS: M = 1:1 MOUNT: C OR CS

(B) OBJECTIVE LENS: ANY C OR CS MOUNT FIBER TYPE: FLEXIBLE BUNDLE RELAY LENS: M = 1:1 MOUNT: C OR CS

FIGURE 4-33 Rigid and flexible fiber optic lenses

be used when no other lens–camera system will solve the problem. Fiber-optic lenses are expensive and available in rigid or flexible configurations (Figure 4-33).

In the complete fiber-optic lens, the fiber bundle is to be preceded by an objective lens, FFL or other, which focuses the scene onto the front end of the bundle and

followed by a relay lens that focuses the image at the rear end of the bundle onto the sensor (Figure 4-34).

Fiber-optic lenses are used in security applications for viewing through thick walls (or ceilings), or any installa- tion where the camera must be a few inches to several feet away from the front lens, for example the camera

G G

COHERENT

6 to 12 INCHES LONG

FIBER OPTIC BUNDLE

OBJECTIVE LENSRELAY LENS

IMAGE ON

SENSOR

CAMERA SENSOR

SCENE

SCENE IMAGE

FIBER OPTIC

OUTPUT SCENE

FIGURE 4-34 Fiber-optic lens configuration

Lenses and Optics 103

on an accessible side of a wall and the front of the lens on the inaccessible scene side. In this situation the lens is a foot away from the camera sensor. Chapter 18 shows how coherent fiber-optic lenses are used in covert security applications.

The bore scope lens is another class of viewing optics for video cameras (Figure 4-35). This lens has a rigid tube of 6−30 inches long and a diameter of 0�04–0�5 inches. The two generic designs, single rod lens and multiple small lenses, transmit the image from the front objective lens to the rear lens and onto a camera sensor. The single rod lens uses a unique graded index (GRIN) glass rod, to refocus the image along its length.

Bore scope lenses can only transmit a small amount of light because of the small rod or lens diameters. This results in high f-numbers, typically f/11 to f/30. The slow speed limits the bore scope application to well-illuminated environments and sensitive cameras. The image quality of the bore scope lens is better than that of the fiber- optic lens, since it uses all glass lenses. Figure 4-35 shows the diagram of a GRIN bore scope lens 0.125 inches in diameter and 12 inches long, and an all-lens bore scope with a diameter of 0.187 inches and a length of 18 inches. The latter has a mirror at the tip to allow viewing at right- angles to the lens axis.

4.7.3 Bi-Focal, Tri-Focal Image Splitting Optics

A lens for imaging two independent scenes onto a single video camera is called an image-splitting or bi-focal lens. The split-image lens has two female C or CS lens ports for two objective lenses. The lens views two different scenes with two separate lenses, and combines the scenes onto one camera sensor (Figure 4-36).

Each of the two objective lenses can have the same or different FLs and will correspondingly produce the same or different magnifications. The split-image lens accom- plishes this with only one camera. Depending on the orien- tation of the bifocal lens system on the camera, the image is split either vertically or horizontally. Any fixed-focus, pinhole, vari-focal, zoom, or other lens that mechanically fits onto the C or CS mount can be used. The adjustable mirror mounted on the side lens allows the camera to look in almost any direction. This external mirror can point at the same scene as the front lens. In this case, if the front lens is a wide-angle lens (4 mm FL) and the side lens is a narrow-angle lens (50 mm FL), a bifocal lens system results: one camera views a wide-field and narrow- field simultaneously (Figure 4-36). Note that the horizontal scene FOV covered by each lens is one-half of the total lens FOV. For example, with the 4 mm and 50 mm FL lenses on a 1/3-inch camera and a vertical split (as shown), the

CAMERA

RELAY LENSES

WORKING LENGTH

CAMERA

RIGHT ANGLE

STRAIGHT FRONT LOOKING

RELAY LENS

GRADIENT INDEX LENS

SINGLE GRADED INDEX (GRIN) LENS

MULTIPLE DISCRETE LENSES

OBJECTIVE SECTION

SECTION OBJECTIVE

SENSOR

VIEWING HEAD

SENSOR

VIEWING HEAD

FIGURE 4-35 Bore scope lens system

104 CCTV Surveillance

HORIZONTAL SPLIT

VERTICAL SPLIT

M = ADJUSTABLE MIRROR

NARROW FOV

WIDE FOV

SCENE B

SCENE A

A B

SCENE A

SCENE B

MOVEABLE MIRROR

FIGURE 4-36 Bi-focal split-image optics

4 mm lens displays a 30 × 45 feet scene, and the 50-mm lens displays a 2�4 × 3�6 feet scene at a distance of 50 feet. The horizontal FOV of each lens has been reduced by one- half of what each lens would see if the lens were mounted directly onto the camera (60 × 45 feet for the 4 mm lens and 4.8 × 3.6 feet for the 50 mm lens). By rotating the split- image lens 90� about the camera optical axis a horizontal split is obtained. In this case the vertical FOV is halved. It should be noted that the bifocal lens inverts the picture on the monitor, a condition that is simply corrected by inverting the camera.

A three-way or tri-split optical image-splitting lens views three scenes (Figure 4-37). The tri-split lens provides the ability to view three different scenes with the same or different magnifications with one camera. Each scene occupies one-third of the monitor screen. Adjustable optics on the lens permit changing the pointing elevation angle of the three front lenses so that they can look close-in for short hallway applications and all the way out (near hor- izontal) for long hallways. Like the bi-split lens, this lens inverts the monitor image, which is corrected by inverting the camera. Both the bi-split and the tri-split lenses work on 1/4-, 1/3-, or 1/2-inch camera formats.

The image splitting is accomplished without electronic splitters and is useful when only one camera is installed but two or three scenes need to be monitored.

4.7.4 Right-Angle Lens

The right-angle lens permits mounting a camera parallel to a wall or ceiling while the lens views a scene at 90� to the camera axis and wall or ceiling (Figure 4-38).

When space is limited behind a wall, a ceiling, in an automatic teller machine (ATM) or an elevator cab, the right-angle lens is a solution. The right-angle optical sys- tem permits use of wide-angle lenses (2.6 mm, 110� FOV) looking at right angles to the camera axis. This cannot be accomplished by using a mirror and a wide-angle lens directly on the camera since the entire scene will not be reflected by the mirror to the lens on the camera. The edges of the scene will not appear on the monitor because of picture vignetting (Figure 4-39).

The right-angle adapter permits the use of any FL lens that will mechanically fit into its C or CS mount and works with 1/4-, 1/3-, or 1/2-inch camera formats.

Lenses and Optics 105

SCENE 1

SCENE 2

SCENE 3

FRONT LENS SECTION MOVES

UP AND DOWN TO CHANGE VERTICAL POINTING ANGLE

MONITOR DISPLAY

CAMERA

LENS 1

LENS 2 LENS 3

2 3

FIGURE 4-37 Tri-split lens views three scenes

NARROW OR WIDE ANGLE

C OR CS MOUNT

ANY C OR CS MOUNT LENS

CAMERA

RIGHT ANGLE RELAY LENS

MONITOR DISPLAY

FULL SCENE

FIGURE 4-38 Right angle lens

106 CCTV Surveillance

WIDE-ANGLE LENS

CAMERA

SENSOR

THIS PART OF SCENE IS BLOCKED BY LENS

THIS PART OF SCENE IS NOT REFLECTED OFF MIRROR

FRONT SURFACE MIRROR

FIGURE 4-39 Picture vignetting from wide-angle lens and mirror

4.7.5 Relay Lens

The relay lens is used to transfer a scene image focused by any standard lens, fiber-optic or bore scope lens onto the camera sensor (Figure 4-40).

The relay lens must always be used with some other objective lens and does not produce an image in and of itself. When used at the fiber bundle output end, a fiber-optic lens re-images the scene onto the sensor. When incorporated into split-image or right-angle optics,

RELAY

LENS OBJECTIVE LENS

CAMERA

FOCUSED SCENE IMAGE

IRIS FOCUSED SCENE IMAGE

SCENE

AUTO IRIS RELAY LENS WITH C MOUNT LENS

FIGURE 4-40 Relay lens adapter

Lenses and Optics 107

it re-images the “split” scene or right-angle scene onto the sensor. The relay lens can be used with a standard FFL, pinhole, zoom, or other lens as a lens extender with unit magnification (M = 1), for the purpose of optically “moving” the sensor out in front of the camera.

4.8 COMMENTS, CHECKLIST AND QUESTIONS

• A standard objective lens inverts the picture image, and the video camera electronics re-invert the picture so that it is displayed right-side-up on the monitor.

• The 25 mm FL lens is considered the standard or ref- erence lens for the 1-inch (actually 16 mm diagonal) format sensor. This lens–camera combination is defined to have a magnification of M = 1 and is similar to the normal FOV of the human eye. The standard lens for a 2/3-inch format sensor is 16 mm; for a 1/2-inch sensor is 12.5 mm; for a 1/3-inch sensor is 8 mm; and for a 1/4- inch sensor is 6 mm. All these combinations produce a magnification M = 1. They all have the same angular FOV and therefore view the same size scene.

• A short-FL lens has a wide FOV (Table 4-3; 4.8 mm, 1/2- inch sensor sees a 13.4 feet wide × 10.0 feet high scene at 10 feet).

• A long-FL lens has a narrow FOV (Table 4-3; 75 mm, 1/2-inch sensor sees a 4.3 feet wide × 3.2 feet high scene at 50 feet).

• To determine what FOV is required for an application, consult Tables 4-1, 4-2, 4-3, 4-4 and the Lens Finder Kit.

• If the exact FOV desired cannot be obtained with an FFL, use a vari-focal lens.

• Does the application require a manual or motorized zoom lens, a pan-tilt mount?

• Is the scene lighting constant or widely varying? Is a manual or automatic iris required?

• What is the camera format (1/4-, 1/3-, 1/2-inch)? • What type of camera lens mount: C, CS, mini 11 mm,

12 mm, 13 mm, bayonet, or other?

4.9 SUMMARY

The task of choosing the right lens for a security applica- tion is an important aspect in designing a video security

system. The large variety of focal lengths and lens types make the proper lens choice a challenging one. The lens tables and Lens Finder Kit provide convenient tools for choosing the optimum FL lens to be used and the result- ing angular FOV obtained with the chosen sensor size.

The common FFL lenses used in most video systems have FLs in the range of 2.8–75 mm. Super wide-angle applications may use a 2.1 mm FL. Super telephoto appli- cations may use FLs from 100 to 300 mm. Most in the range of 2.8–75 mm are available with a manual or auto- matic iris.

Vari-focal lenses are often chosen when the exact FL desired cannot be obtained with the FFL. The vari-focal lens can “fine tune” the focal length exactly to obtain the angular FOV required. Vari-focal lenses are available in manual and auto-iris configurations. Vari-focal lenses must be re-focused when their FL is changed.

Zoom lenses are used when the camera and lens must be scanned over a large scene area and the magnifica- tion of the scene must be changed. This is accomplished by mounting the camera-lens on a pan-tilt platform capa- ble of remotely panning and tilting the camera-lens assem- bly and zooming the zoom lens. Zoom lenses are available with FLs from 8 to 200 mm with zoom ratios from 6 to 50 to 1. The zoom lens is available with a manual or auto- matic iris.

When the video security application requires that the camera and lens be hidden, covert pinhole lenses and mini-pinhole lenses are used. The pinhole lenses are mounted to cameras having a C or CS mounts. The mini- pinhole lenses are mounted directly onto a small sin- gle board camera (with or without housing) and hidden behind walls or ceilings or mounted into common objects: PIR motion sensor, clock, emergency light, sprinkler head, etc.). Chapter 18 describes covert video lenses and systems in more detail.

Special lenses like the bi- and tri-split, fiber-optic or bore scope lenses are only used when other simpler techniques can not be used.

The newly implemented 360� panoramic lens is used with a computer system and can view a 360� horizontal FOV and up to a 90� vertical FOV. This lens has taken an important place in digital video surveillance systems. The computer transforms the complex donut-shaped image into a useful rectangular image on the monitor.