focal length isn’t a measure of how long or short a lens is physically, but the distance in millimeters from the optical center of a lens to the imaging sensor when the lens is focused at infinity.
For a camera with a full-frame sensor, for example, a standard lens (one that gives a similar perspective to the human eye) is 50mm.
Lenses with focal lengths less than 50mm are referred to as wide-angles – simply because they have a wider angle of view. Lenses with focal lengths greater than 50mm are known as telephotos, and these offer greater magnification thanks to their much narrower angle of view.
the angle of view is the amount of a scene that a lens can take in,
measured in degrees. For instance, a fisheye lens may offer an extremely
wide 180° angle of view, meaning that it
can capture everything in
front of it (and to each side).
A 200mm lens, on the other hand, will
offer a much narrower
angle of view of 12.3°. This allows you to fill
the frame with a
considerably smaller amount of the scene that you’re
trying to
photograph.
Full-frame sensors get their name because, at 36x24mm, they have similar dimensions to a frame of 35mm film. This means that they capture the full angle of view offered by a lens that’s been designed for a film or full-frame camera.
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