• Originally Posted by benjiboy
According to Tiffen the filter factor of their 25A is 3, and they recommend you open up two stops to compensate for the light loss.
Wait a minute; that makes no sense. For one, the two numbers don't match each other. If you were supposed to open up two stops, the filter factor would be 4. If the filter factor was 3, you would open up 1-2/3 stops. For another, the filter factor for the filter is 8, and you are supposed to open up 3 stops (2x2x2=8).

Originally Posted by Bruce Osgood
Is a filter factor the same as stops? I didn't think so. If a filter has a factor of 3 wouldn't you open up 4 stops (1, 2, 4)?
If a filter has a factor of 3, you'd open up 1-2/3 stops.

The way I figure it out in my head is to divide 100 percent by the filter factor; that is how much light makes it through. Then I count stops to compensate. For example, with a filter factor of 3, one third of the light makes it through. Adding one stop would half 100 percent, to compensate down for 50 percent. Adding another stop would cut 50 percent in half, to compensate down to 25 percent. Therefore you open up between one and two stops. Since the light loss is slightly more than halfway between 50 and 75 percent, 1-2/3 is close enough.

If the filter factor was 8, I'd take 1/8 of 100 percent, to get 12.5 percent. Cut 100 percent in half once to get 50 percent. Cut 50 percent in half once to get to 25 percent. Cut 25 percent in half once to get 12.5 percent. Voila. Three stops makes up for the light loss.