When you use more than one flash, you can adjust each light individually. If you use speedlights and your camera maker’s wireless TTL functionality, you can do that in two ways:
- Canon: relative, using ratios. As in “A:B = 4:1” meaning A is 4x (two stops) lighter than B
- Nikon: absolute, by adjusting individually. E.g. “A = No adjustment; B = -2 stops adjustment”.
An example of this using two Canon flashes.
Ratio between key light (face) and background light: 1:2
2:1 (you are getting the hand signals by now I presume):
The preference is yours: mine is 2:1. But that is largely a matter of taste.
If you use manual flash and radio triggers, it is conceptually easier, since you set up each flash by itself, independent of other flashes. Ratios are a little bit trickier to get your head around, because it is not immediately clear which flash will get darker and which flash lighter, and to what extent. So trial and error will be required. Either way, key point is that you should think carefully about how bright each flash is, in a multi-flash setup.