Francis found the appropriate equation for film exposure in the IES
handbook.  There isn't an exact relation, but the following formula
can be used to get an approximate answer for 35mm photography:

	Radiance EXPOSURE = K * T * S / f^2

		where:
			T = exposure time (in seconds)
			S = film speed (ISO ASA)
			f = f-stop
			K = 2.81 (conversion factor 179*PI/200)

This came from the IES Lighting Handbook, 1987 Application Volume, section 11,
page 24.

So, if you were trying to produce an image as it would appear shot at
1/60 sec. on ISO ASA 100 (DIN 21) film at f-4, you would apply pfilt
thusly:

	pfilt -1 -e `ev "2.81*1/60*100/4^2"` raw.pic > fin.pic
