Manual flash calculation question

[Andre Reinders]

Now I know that if you have a manual flash with guide number 110 (as an example) that at a distance of 10 feet to the subject - you should use f/11 as a starting point for exposure.

Now the question is - if the flash has variable power output and you dial it to 1/2 - does that mean the guide number becomes 55, so you would use an aperture of f/5.6? (at 10 feet) or does the 1/2 power translate to 1 stop more light required - so f/11?

I would guess the first - but that is why I am asking.......

Thanks!

BTW - I just bought a Vivitar 285 .....

[Julio1fer]

Usually 1/2 means half the power, therefore you would have to open 1 stop more with 1/2 than with full power.

[ImageMaker]

Exactly.  For guide numbers specifically, half the power means the guide number is about 71% (divide by the same distance to get one stop larger, so square root of 1/2).  Your hypothetical GN 110 at full power would become about 79 at half -- 80 is close enough.

[Andre Reinders]

Thanks - that makes calculations easier!