scott wrote:

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?

Focal length is exactly what it sounds like -- the distance from the lens
to focal (film or sensor) plane. So yes, a basic 200mm lens would be 20cm
from the film when focused at infinity.
Of course, we want things compact so more sophisticated optics are used to
compress the physical length of the lens. Zoom lenses are more sophisticated
again, but the focal lengths still refer to a simple lens.

MJ