"scott" wrote in message news:NfHEc.276$WO5.171@newsfe3-gui...
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

My understanding of this is that the stated focal length of a camera
lens is the distance a pinhole needs to be from the film plane to
produce an image the same size on 35mm film.
What this means is that as the film (or sensor) size goes down (like
the smaller dslr aps sized sensors and tiny p&s 2/3 sensors) the
realative focal length goes up and as the film size goes up
(120/meduim format and up to large format and beyond for plate
cameras) the realtive focal length goes down.

Just my 2c