Calculating Field of View
Rectilinear Lenses
The field of view of a rectilinear lens focused at infinity is very easy to calculate using simple trigonometry. It's given by:
FOV (rectilinear) = 2 * arctan (frame size/(focal length * 2))
Here "frame size" refers to the dimension of the frame in the direction of the FOV, so for 35mm (which is 24mm x 36mm), frame size is 36mm for the horizontal FOV, 24mm for the vertical FOV and 43.25mm for the diagonal FOV.
Below is a calculator for FOV in both angular and linear terms. The angular field of view assumes infinity focus, and the linear field of view is also based on that, which isn't technically correct, but as described below the corrections for closer focusing aren't significant until you get into the macro range. The "digital multiplier" is 1.6x for most Canon consumer DSLRs, 1.3x for the EOS 1D, 1x for the EOS 1Ds, 1.5x for most Pentax, Nikon and Sony DSLRs and 2x for most Olympus DSLRs.