Question: how does light travel through a camera

Derek McKay-Bukowski answered on 16 Jun 2011:

There are things a camera does. It focuses the light into an image. Then it detects that image.

When light is passing through the camera, it is doing the first of these. The process of focusing is usually done with lenses. I say usually, because that’s what most commercial cameras use. However it is also possible to use mirrors (especially in astronomical telescopes) or a very small hole (such as in a pin-hole camera).

In the case of a lens, the lens focuses the light so that rays from a particular direction converge on a single point somewhere inside the camera. This is where either the film or the CCD chip goes (both of which are types of detectors). This usually means that the light has to be focused in such a way that the light converges somewhere on a flat surface (where the chip or film is mounted) regardless of where in the scene it started from.

By changing the rate at which the light converges (by adjusting the lenses) you can change the exact point at which the light focuses (either in front of or behind the plane). When you “adjust the focus” this is what you are doing… in a way you are “calibrating” the camera.

Furthermore, if you have a complex lens system, you can design it in such a way that the focal plane doesn’t change, but the scale of the image on that plane does. This is what a zoom lens does.

The bigger the aperture, the more you need to control the focus. This is because larger apertures are more picky about their focal plane. In photography, this is know as depth-of-field. Because mobile telephones have small lenses they often don’t need to be focused at all!

When the light strikes the sensor, it stops travelling. What happens then is that either it blackens a small part of the film (this is a light-sensitive chemical reaction) or it causes a small charge in a silicon ship (this is what a CCD chip does). By processing the film chemically, or the chip electronically, the image can be recovered.