Originally posted by baro-nite Um, the latter.

1 stop from f/4.5 is either f/6.3 or f/6.7. I say either because of Pentax's strange use of 4.5 to signify both the 1/3 stop past 4.0, and the 1/2 stop past 4.0. So 1.5 stops from f/4.5 is either f/8 or a 1/6 of a stop up from that. That's pretty small for the Q, which is why I asked.

baro-nite is correct. One stop difference is either twice or half the aperture area (size of the hole in the aperture blades -- 2x the aperture area lets in 2x the mount of light) from your starting point. To calculate 1 stop difference in aperture value, you would either multiply or divide by a factor of 1.4x (sq root of 2 rounded) to go 1 stop slower or faster respectively. You can confirm that this is valid as the "standard' full stop values increment by a factor of 1.4x -- f1, f1.4, f2, f2.8, f4, f5.6. f8, etc.

f4.5 stopped down 1 full stop would be f4.5 x 1.4 = f6.3.

To calculate 1.5 stops, you'd use a 1.7x multiplication factor (sq root of 3 rounded), and for 2 stops, the factor is 2x (sq root of 4).

I do these calculations pretty much on a daily basis since I use TCs extensively (some would say excessively

) and know that I need to keep my max apertures at f8 or faster to get reasonable AF performance with my DSLR bodies.

Scott