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

When you see a 200mm lens, what is the 200mm actually a measure of?

The focal length of a thin lens is the distance from
the lens (or rather the centre of the lens) to the
picture plane if the object you are imaging is at
infinity.

The focal lenghth of a thick lens (such as you find in
your camera) is defined as the focal length of an imaginary
thin lens that gives the same size picture in the focal plane.

And yes - angles would be nice. But - to define the view angle
you must also provide the sensor/film size. So - the actual
meassure for the lens is the focal length and not any angle.

I agree though that it would be nice, to be able to compare
cameras easier, that the max view angle would be given instead.
But evan that is not perfect as the sensors migh come in different
shapes, e.g. 1:1, 2:3, 3:4, ...


/Roland