PHOTOGRAPHIC CAMERAS AND PROJECTORS 2

April 5, 1995

One Minute Papers - Questions and Answers

Why do people in flash pictures have "red eye"? How do cameras try to solve that problem?

When light from the flash illuminates peoples eyes, that light focuses onto small spots on their retinas. Most of the light is absorbed, by a small amount of red light reflects. Because the lens focused light from the flash onto a particular spot on the retina, the returning light is directed back toward the flash. The camera records this returning red light and eyes appear bright red. To reduce the effect, some flashes emit an early pulse of light. People's iris shrink in response to this light and allow less light to go into and out of their eyes.

Why is film ruined when it is exposed to light?

Photographic film chemically records information about the light that it has absorbed. Normally, this light was projected on it by a lens and formed a clear, sharp pattern of the scene in front of the camera. However, if light strikes the film uniformly, the information recorded on the film will have nothing to do with an image. The entire sheet of film will record intense exposure to light and will have no structure on its chemical record.

Why are their various types of film (speed, purposes, etc.)?

The different speeds of film have to do with how light sensitive the film emulsion is. A portion of the surface of a high speed film will record exposure to light when only a few particles of light (photons) reach it. In contrast, a low speed film requires more photons per square millimeter to undergo the chemical changes of exposure. So high speed film can take pictures with less light than low speed film. There is a trade-off, however. High speed films are more grainy and have less resolution than low speed films. Thus photographs that you would like to enlarge should be taken with relatively slow film.