A simple lens can be used to form an image on the film plane of a camera. The lens has to be positioned at the correct distance from the film so that the image does fall on the film rather than in front of it or behind it. When you (or the auto-focus mechanism) focus the camera, you are ensuring that the real image formed by the lens is on the film plane. If the object is a very long way away, the image will be formed very close to the focal plane of the lens. The magnification of the image is then
l ' / l =
f ' / l.This means that for distant objects the linear magnification is directly proportional to the focal length of the lens.
For conventional "35 mm" cameras, a lens of 50 mm focal length is often the standard lens, a lens of 28 mm focal length is a wide-angle lens, and a lens of 300 mm focal length gives a larger image of a distant object.
What is the angular field of view of a camera which uses a 50 mm focal length lens to form an image on a section of film of length 35 mm?
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