Originally Posted by CPorter
A 3x factor does not add up mathematically to a 2 stop compensation-----a 2 stop compensation is a 4x factor, adding twice the amount of light with one stop, then adding twice the amount of light again with two stops.

21 = 2x factor
22 = 4x factor
23 = 8x factor

A 3x factor is an intermediate factor that adds 1 2/3 stops.

It's the 1:2 ratio that exists between each f/stop and each shutter speed on the lens. Opening up one stop adds twice the light, stopping down one stop cuts the light in half, therefore, if you open up 2 stops with the 25A filter, you are adding a 4x factor to the film.
Right.

I think they only said 2 stops as a suggestion, since you can't practically set 1 2/3 stop.

But you can carefully note meter reading to nearest 1/3 stop, add 1 2/3 stop filter factor. Then sometimes the correct exposure (including filter factor) will fall exactly. Or you can include filter factor in the EI that you set on a handheld meter.

When I go out, I often pick the filter in advance and use it the whole day. I set the EI on my handheld meter to include the filter factor. When I go inside, then I take the filter off and set EI on the meter to rated speed.