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

			T = exposure time (in seconds)
			S = film speed (ISO)
			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 ASA 100 (ISO 21) film at f-4, you would apply pfilt thusly:

	pfilt -1 -e `ev "2.81*1/60*21/4^2"` raw.pic > fin.pic