PHOTOGRAPHIC CAMERAS AND PROJECTORS 1

April 3, 1995

One Minute Papers - Questions and Answers

How does the camera know (measure) what the distance is to the object?

Modern cameras use a variety of techniques to find the distance to objects. Some cameras bounce sound off of the objects and time how long it takes for the echo to return. Others observe the central portion of the image (presumably the object) from two vantage points simultaneous and then adjust the angles at which those two observations are made until the images overlap. This rangefinder technique is the one you use to sense distance with your eyes. You view the object through each eye and adjust the angles of view until the two images overlap (in your brain). At that point, you can tell how far away the object is by how crossed or uncrossed your eyes are. A rangefinder camera has two small viewing windows and lenses to look at the object, just as you have two eyes to look at the object. Finally, some cameras don't really measure the distance to the object but instead adjust the lens until it forms the sharpest possible image. A sharp image has the highest possible contrast while an out-of-focus image will have relatively low contrast. The cameras adjust the lens until the light striking a sensor exhibits maximal contrast (brightest bright spots and darkest dark spots).

Does your pupil opening and closing have anything to do with it focusing on a more distant object?

The size of your pupil does not depend on the distance to an object. It depends only on how bright the scene in front of you is. But the size of your pupil does affect your ability to focus. When it is relatively dark and your pupil is wide open, the whole lens of your eye is involved in light gathering. Focusing becomes very critical and you have very little depth of focus. Moreover, if your lens isn't perfect, you will see things as blurry. But when it is bright out and your pupil is small, you are only using the center portion of your lens and everything is in focus. That's why it is harder to focus at night than during the day. When you squint, you are artificially shrinking the effective diameter of the lens in your eye and increasing your depth of focus. Unfortunately, you are also reducing the amount of light that reaches your eye. If you look through a pin hole in a sheet of paper, you will find everything in focus, although it will appear very dim.

Is the eye similar to a camera?

Yes, your eye is exactly like a camera, except that the real image forms on your light sensitive retina rather than on a sheet of film. The lens bends light to a focus on the retina. If you are nearsighted and can only see nearby objects clearly, then your lens is too strong and bends light too much. Light from a distant object focuses before reaching your retina. If you are farsighted and can only see distant objects clearly, then your lens is too weak and bends light too little. Light from a nearby object doesn't reach a focus by the time it strikes your retina. It would focus beyond your retina, if it could continue on through space.