In article <31CD8D0E.7B77@bu.edu>, ma@bu.edu says...

>If the filter factor says X, where X is a number printed on the holder,
>say, what is the compensation in terms of f stops (or exposure time)?

Filter factors are nearly absurd, for practical purposes, IM(NS)HO,
and should be replaced with simple number of stops to be compensated.
Anyway, the Filter Factor is the number 2 raised by the power of the
number of stops compensation required, as in:
Red Filter Factor = 8 = 2 to the third power (so, 3-stops compensation),
Orange Filter Factor = 4 = 2 squared (so, 2-stops compensation),
Dark Yellow FF = 2 = 2 to the first power (so, 1-stop compensation).
Things get fun with fractional filter factors, and ones that are not whole-number powers of 2, requiring a guestimate.
Hope This Helps