As Adam notes, that chart is the horizontal field of view, so if you want diagonal, it won't really help.

Rather than using a table, if you have a scientific calculator handy, then the math is pretty simple.

The APS-C sensor used in Pentax bodies has dimensions of approximately 23.7x15.7 mm, with a diagonal of approximately 28.4 mm. This is for the K-5; sensors in the other cameras might be very slightly different (probably within 0.1 mm).

The formula for calculating APS-C diagonal field of view (DFOV) on a rectilinear lens is simply:

DFOV = 2 * arctan (14.2 / f)

Where "f" is the

__actual__ focal length of the lens, in mm,

__not__ adjusted for any crop factor. The "14.2" represents half of the diagonal dimension of the sensor, so if you want (for example) a horizontal field of view, use half the horizontal dimension in its place. You can substitute dimensions taken from full-frame, 4/3, or whatever you like, as long as you can figure out what the true dimensions are. And if you have your calculator's angle measurement set to degrees, your DFOV output will also be in degrees.

(On a calculator, the "arctan" function is usually marked "Tan-1" with the "-1" superscript, but I can't figure out how to show that superscript in this text!)

So for a 70mm focal length, I'd do this (reading from left to right):

DFOV = 2 * arctan (14.2/70) = 2 * arctan (0.203) = 2 * 11.5 = 23.0 degrees

And so on!

This will give you a good idea of what a lens's field of view will be. But it won't necessarily be 100% accurate -- manufacturers don't exactly quote precise focal length numbers (e.g., a "20mm" lens might actually be 1-2 mm off from that), and focal length might vary as you focus from near to far, sometimes by quite a lot, depending on the lens.

*
Last edited by Quicksand; 05-06-2011 at 11:23 AM.
*