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.

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

TIA

Scott