"scott" writes:

When you see a 200mm lens, what is the 200mm actually a measure of?
I assume it's something to do with how much "zoom" there is, but
surely measuring the angle that is shown by the image would be
better? Eg a wide-angle lens would be a 90 degree field of view but
a telephoto one would be maybe 5 or 10 degrees.

Focal length is a real optical property, basic to the lens. Angle of
view isn't; it's the result of cropping the image circle to some
particular film format -- and assumes that the lens throws an image
circle big enough for that film format. (The image circle isn't
simply a function of the focal length, for real-world complex lens
systems like what we actually use on cameras.)

The focal length is needed to calculate aperture, the depth of field,
and a number of other things. So going just by angle of view isn't
really enough.

Is it possible to work out the field of view in degrees from the focal
length?

Yes, for some particular given film format.
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