How can I keep exposure and white balance consistent when changing aperture with a speedlight?

Let’s pull the question apart...

Assuming a full strobe lit scene...

how to maintain exposure while changing f stop?

The only other variables in play are ISO and strobe power. So if you go from f/4 to f/8, you’d need to correspondingly either raise your ISO two stops or double your strobe power, twice.

how to maintain white balance while changing strobe power

Strobes naturally have a different white balance through the power range. The best thing you can do is use customs white balance and redo it after changing power output.

So, putting it together, to maintain exposure and white balance, you’d need to increase ISO or strobe power, or both, and then redo your white balance setting.

——-

Assuming a mixed lighting scenario...

how to maintain exposure while changing f stop

Let’s assume that you’re mixing natural and artificial light and are using the strobe as fill. If you go from f/4 to f/11, then your strobe must get 2^3 times more powerful to get the same amount of fill flash off the subject and into the camera.

But, also keep in mind that you’ve dropped your ambient capture as well. So you’ll have to lengthen the shutter speed by 3 stops as well.

how to maintain white balance

Well, if you’re mixing strobe and ambient, you’ve probably already placed a gel or filter over the strobe to balance it to your ambient. Not much should need to change. If your strobe swings so wildly at different power outputs, you may need to change filters/gels.

——

A note on the inverse square law

In both scenarios, increasing strobe power is a solution for getting correct subject exposure when decreasing your f stop. Keep in mind that this will spill more light into your background. It may be necessary to instead mess with not only the lights power but also it’s placement distance from the subject.