VA 5

VA--- Week 5

Part 1: Answer the questions and use your own word to finish (200-250 words)

QUESTION 1: The following photographs show real scenes, but they seem unreal.  If you follow the links, you can find similar works by the same artist. Carefully examine one of the artist's works, and then discuss: Why were you confused: What assumption did your visual brain automatically make? Why did your brain have to make an assumption? Why are those assumptions usually valid? How did the artist break that assumption?

 Question Mark, Kumi YamashitaMore examples by the same artist:

· http://www.kumiyamashita.com/light-shadow/

· https://vimeo.com/8015989  (*this is less relevant, but still fun)

Shigeo Fukuda

Part 2 read the information and answer the question

This week, we're going to look at how our eye (or a camera) creates a 2D image of the 3D world. Then we'll discuss how our brain reverses this process, creating the perception of a 3D world from a 2D image.  

The formation of a 2D image.

A light source like the sun emits light rays in all directions. When these light rays hit a surface, they bounce off in all directions1. We use these reflected light rays to see the world around us, but to do this our eyes must select a single light ray (or small bundle of rays) from each point on the object.  To function, an eye (or a camera) must form an image by mapping one point in space to one point on the eye's retina (or camera's sensor). The simplest way to form an image is with a pinhole.   The pinhole only lets in one light ray from each point in space, as the diagram below shows.  When light hits the top of the tree, for example, the top leaf reflects the light in all directions. Only one of these light rays, shown in red, intersects the pinhole and enters the box. By being so small, the pinhole selects only one light ray from each direction in space.  With a pinhole camera, everything in the scene is in focus; near objects and distant objects are both crisply defined. Pinhole Camera, Wikipedia Pinholes were used in the very first camera, the "camera obscura".  A camera obscura consists of a black box that eliminates all light except the light entering through a very small hole.    As the figure above shows, the image formed by the camera is upside down. The same is true of the retinal image: the image is upside down, and our brain flips the image rightside up.  Watch this movie about  the photographer Abelardo Morell to learn more about the  camera obscura (he sometimes adds a lens to the pinhole for a reason we'll discuss next).  Optional: You can learn more about Morell's work here.  Also, you can find many videos of people making camera obscuras on the internet, here's a small sampling:  HowCast, Kodak , National Geographic , PBS , and, best of all, the camper obscura  One problem with the pinhole camera is that because the opening is so small, it lets in very little light. For a brighter image, you need a lens, which can collect a bundle of light rays and focus them on a point. Watch the first minute of this video to see how lenses work.  While a lense allows in more light than a pinhole camera, it introduces new problems as explained in the second half of the lens video. We'll discuss two of these problems later (spherical aberration and chromatic aberration), but for now we'll examine the problem of focus.  1 There are some exceptions to this: some surfaces are transparent and light passes through them, other surfaces are shiny and light bounces off them in a particular direction. We'll get into these exceptions later.

The human eye has a lens that focuses light on the retina. (The lens is the ellipse in the center of these cross-sections of the eye.) There are muscles attached to the lens that can control its shape and this allows us to focus either on near objects (when the lens is thick) or far objects (when the lens is thin).  If you'd like to learn more  watch the first 5 minutes of this video .  When we're focused on something near, everything in the distance will be blurry.  When we're focused on something far, everything near will be blurry. Normally, you don't notice the blur because you're only paying attention to the thing you're focused on.  https://library.creativecow.net/terry_todd/depth_of_field_converters/1

The range of things that are in focus is called the "depth of field", and it is determined by the size of the eye's pupil. As we just learned, in a pinhole camera, everything is in focus and the depth of field is infinite.  We have the largest depth of field when our pupil is maximally constricted.   Our pupil constricts in bright light  (at 40 sec).  So,  the brighter the light,  the smaller the pupil ,  the larger the depth of field, the better the overall focus. This is one of the reasons it's easier to read with a bright light - it makes it easier to focus on the print.  Squinting also reduces the effective size of the pupil; that's why squinting helps you see better.  The optics are the same in photography, but the camera's "pupil" is called the aperture (or f-stop). The diagram below shows the relationship between the f-stop and the depth of field. Notice again that when the aperture is large (left) only a few pencils ae in focus. When the aperture is small (right) all of the pencils are in focus.

If you find this interesting, keep reading... (this gets a little off topic). Depth of field is also reduced by magnification. When you take a close-up photograph of a very small object, the depth of field is very shallow.  Photographers can make a real scene look like a miniature by blurring the foreground and background of the image.  Notice how the scene appears to be a toy replica when depth of field is very shallow.

The fact that we can interpret the shallow depth of field as a sign of magnification shows that we all understand this phenomenon at some level. This is another example of implicit knowledge - something we don't know we know.

· We've seen that the lens in our eye causes us to have a limited depth of field: the object we are looking at will be in focus, but nearer and further objects will be blurred.  Our brains make use of this fact to infer depth.  In the photo, "Cotton Mill Girl" the subject of the photo is in focus, but the more distant girl and is blurry.  Similarly in the photo "V-J Day in Times Square", the subject of the photo, the kissing couple, is in focus, but the distant buildings are blurry.   Source Source Paintings do not have the same limitation as photographs, and everything in the work can be crisp and detailed. Nonetheless, some artists seem to simulate a limited depth of field in their paintings.  This technique directs the viewer's attention to the subject of the painting, in this case, the woman's face.  Artists sometimes blur an entire image. This might suggest a quickly changing environment that we struggle to bring into focus or it might suggest that we are unable to get a clear understanding of the subject.

· Answer The 3 questions

· 1. Images made by a pinhole camera are

· A. Bright

· B. Blurry

· C. Upside down

· D. all of the above

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· 2. “Depth of field” refers to

· A. the range of distances that are in focus in an image

· B. the size of an image.

· C. the thickness of the lens required to focus a distant object.

3. The lens in the human eye

A. in convex

B. can change shape

C. refracts light

D. all of the above 

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