Reader Michel, referring to a previous post, asks:

Hi Michael, I am struggling with the math of this statement…. “two lights that both say f/5.6 will give you a total of f/8″… Can you clarify?

Sure.

The “main f-numbers” (1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, etc) are the square root of 2 apart (each is 1.41.. times the previous number). This is so that they they are all one stop apart (pi x r squared is the area of a circle). Anyway, math notwithstanding, one stop means double, or half, the amount of light.

So if I were to shoot a photo at f/5.6, and then I repeat it at f/8, I would get half the light in that second picture.

But if at the same time I add a second flash, which doubles the light, I would get net the same amount of light. Two flashes doubles it; one stop higher f-number halves that.

So if you need to set your camera to f/5.6 if you have one flash, you would set it to f/8 if you have two flashes.