Quote Originally Posted by jstraw View Post
When applying more than one factor do you add them or multiply them?
If you are adjusting shutter speeds, you multiply.

If you are adjusting apertures, you add the base 2 logs of them to get the change in stops.

The base 2 logs are the number you "raise" 2 to to get your factor. And they are the stop equivalent of the factor.

Here is a calculator: http://www.miniwebtool.com/log-base-2-calculator/

so for your factor numbers 2.15 and 2.5 the result is 1.1 + 1.3 = 2.4 stops (after rounding off).