A ways back I posted the ">>" marked material below, and Albert Ma
recently responded to it in an e-mail. Below is our correspondence, which we
thought might be of interest:

AM:
>A little while ago, you gave me the conversion of filter factor to aperture
>or f-stop adjustment. The info you gave was as follows:

HTH:
>>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

AM:
>I even made up a little table at your suggestion.
>
>But recently I was reading one of Ansel Adams's books. Indeed, if the
>compensation for exposure due to a filter is done via the shutter speed, the
>filter factor can be applied in a straightforward fashion: just multiply it
>with the shutter speed. for example, if without a filter, the reading is f/8
>at 1/60 sec. Put in a red filter with a factor of 8, then the negative
>should be exposed at f/8 at 8*(1/60) or 1/7.5 sec----approximately 1/8 sec
>will do.
>
>Of course, this is just the equivalent of adjusting the aperture. But
>perhaps, filter factor isn't so absurd. Plus, when using a large format
>camera which is always on a platform, adjusting the shutter speed seems the
>way to compensate for a filter---I suppose Ansel Adams would be more
>concerned with depth of field than with camera shake!
>
>Albert Ma

HTH:
Well done! (Though once you have converted to stops, the number can be
applied to either the shutter speed or aperture.) Why not post all of this
to r.p.misc?
Hope This Helps