How Do I Set The Power On My Speedlight?
This is a page taken from the SimpleSLR Portrait Lighting Guide.
If you are unsure what the power setting of a speedlight should be, you can use this estimation method. Let’s assume we’re using ISO1600, F2.8 and 1/125 (the shutter speed is insignificant unless you exceed the maximum sync speed). Set the ISO and aperture in your speedlight to correspond with those settings. Choose a power setting (with practice you’ll be able to guess quite accurately) and look at the Distance to Subject reading on your speedlight.
This Distance to Subject reading indicates how far your speedlight needs to be from your subject. Please bear in mind that this is just an estimation, and will be affected by factors such as whether your speedlight is directly pointed at your subject, whether a light modifier (eg. umbrella/softbox) is used, and whether a CTO gel is attached.