NOTE: Simplified facts from a Kodak pamphlet follow for your edification!
What makes a lens work?
A camera lens can be made from a piece of glass or plastic which has two
opposite regular surfaces, either both curved or one curved and the other flat.
As the light rays that are reflected from the subject pass through the camera
lens, they are bent. The extent to which they bend is controlled by the
composition of the lens and the curves of the lens surface.
In a properly designed lens, all the light rays from the same part of a subject
will meet at a point behind the lens. A sharp image of the subject is formed at
the point where these light rays meet. The film must be located at this point
for the picture to be in sharp focus.
What is the focal length of a lens and what does it do?
In very simple terms, focal length is the distance between the optical center
of the lens and the film when the lens is focused at infinity. The focal length
of the lens of most adjustable cameras is marked on the lens mount. The focal
length is usually given in millimetres or inches.
There is a direct relationship between the focal length of a lens and the image
size of the subject recorded on film: the longer the focal length, the larger
the image on the film. For example, if Lens A has a focal length of 50 mm, and
Lens B has a focal length of 150 mm. The subject is the same size and the same
distance from the lens in both situations.
In this case, the image produced by Lens B, which has a focal length of 150 mm,
is three times as large as the image produced by Lens A, which has a focal
length of 50 mm.
A lens of long focal length produces a larger image than one of short focal
length. In summary, the size of the image is directly proporational to focal
length. Doubling the focal length doubles the image size assuming a constant
object distance.
posted here by: Andrew Davidhazy, SPAS/RIT
