Depth of Field

• Conversely, for a given film position, there is a range of distance at which all objects have acceptable images on the film. This distance is called the depth of fielddepth of field. Within the depth of field, all objects are reasonably clear on the film.

• Used a lot in filming…(not all distances are clear)

Three factors affect depth of field

f-number

• f-number = focal length / diameter of stop’s hole

example:

f = 50 mm, d = 12.5 mm

Then f/d = 4,

which is written as f/4.

• For a fixed focal length, the f-number tells you directly the size of the stop.

• For the same f-number, the image has the same amount of intensity.

• One can get the same exposure by increase the f-number and the exposure time by the same factors.

– In a bright sunny day… f/11 and 1/60 sec

f/8 and 1/120 sec

Gives the same exposure– If an object is moving, you want to have shorter

exposure time.

Choosing Exposure

Rods and Cones

• Covers an area of 5 cm2. A baseball a mile away gives an image covering one cone.

• Cones: give more precise vision, need strong light; help to see colors; mostly distributed in the center of the retina (fovea).

• Rods: for peripheral and night vision; very sensitive to light; mostly distributed away from fovea.

A summary

62 6460

Normal visionSee infinity See 25cm away

MyopiaHypropia

Presbyopia

Focusingpower

• The total length of the telescope is fo +fe.

Telescope Magnification

• To get a large image, fO must be large.

• To see the image up close, fE must be small.

• The telescope magnification is Negative because the image is inverted.

• A large intermediate image means that it is dimmer. One needs a larger aperture. Thus telescopes are usually specified by their sizes.

eo ff /

Brightness and Lightness

• Brightness: Describe the intensity of the light sources such as sun, candle, – Dark, dim, bright, dazzling…

– Sensation depends on adaptation. The same source may produce different feeling at different time

• Lightness: Describe the appearance of the surfaces:– Black, dark gray, light gray and white..

– Do not depends on adaptation and illumination.

Weber’s Law

• Equal steps in lightness arise from steps of equal ratio in light intensity (logarithmic scale)– 1, 2, 4, 8, 16 … has equal steps in lightness– 1, 2, 3, 4, … does not have equal step (3 is much

closer to 4 than 1 is to 2.

• Limitations: – Beyond certain brightness, your visual system

no longer respond to the increased light. The same thing happens in the opposite limit.

Receptive field

• Refers to a region of retina which will produce a signal to the brain, depending on the pattern of the light falling on it.

Processing Edges

• We rely on edges for information about the the uniform regions between them. When we see something, we first look for edges and then fill in the regions, taking the clue from the behavior noticed at the edges.

• Craik-O’Brien illusion.

Negative Afterimages

• The sensitivity of a given region of retina decreases after it is exposed to a bright light for a period of time—successive lightness contrast.

• Negative afterimage– Look at a white cat with a dark background for

a while, you will see a dark cat on a white background! (last for ~30 sec)

Positive Afterimage

• Persistence of the response positive afterimage. – Last as long as 1/20 sec at low ambient light

levels – As short as 1/50 sec at high light levels.

• If two images are presented in rapid succession , the image will appear as one because your response is too slow to separate and distinguish them.

2-D vs. 3-D world

• A picture is a 2D reduction of the 3D world. How do we appreciate 3D?

We use many different cues (a feature indicating the nature of something perceived)– Accommodation– Convergence– parallax

Monocular vs. Binocular View

• Monocular view: one eye only! Many optical instruments are designed from one eye view.

• Binocular view: two eyes with each seeing a separate picture of the world.– Increase the field of view: fish, rabbits can see 360.– Overlapping view: predators with two eyes in the

front.

How to create 3D using 2D?

• Artists use the following depth cues to convey 3D impression– Size

– Geometrical perspective

– Shadow

– Color

– Sharpness

– Patterns

– Overlay (interposition)

However, they are intrinsically ambiguous, can be interpreted in many ways. We interpret in the most likely possibility.