There has been some discussion of how focal length relates to field of view, so I thought I'd get out Python and make a chart or two. Here's the process I used, so people can catch any mistakes I may have made.

First, for a rectilinear lens, the equation relating focal length (f) to angle of view (a) is well known. The only other parameter it depends on is the image dimension (d), meaning the linear size of the film or sensor. We can choose this to be width, height or diagonal -- I have produced charts with all three.

The formula for the angle in radians is:

a = 2 arctan(d/2f)

(Note that f is technically the "effective focal length", but unless we are doing macro photography this is the same as the focal length stated on the lens.)

For the Pentax APS-C sensor we have these dimensions in millimetres:

w = 23.6

h = 15.7

d = 28.34

Lastly, we can convert radians to degrees by multiplying by 360 / pi.

Using the math library in Python to get the atan function, this reduces to these three equations:

a_w = 114.6 * math.atan(11.8 / f)

a_h = 114.6 * math.atan( 7.85 / f)

a_d = 114.6 * math.atan(14.17 / f)

I plotted them on two scales, for convenience.

I hope this is of use to someone.