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It’s worth noting the difference between flash exposure and ambient light exposure, as this will help you develop versatility in your speedlight portraits, letting you create a wide range of portraits, from dramatically lit portraits to those that appear to be taken with natural light (but are actually achieved with speedlights when the available light is simply just not there).
As described in my SimpleSLR Portrait Lighting Guide, there are 5 factors that affect flash exposure when using speedlights:
- Flash Power
- Flash-to-Subject Distance
- Shutter Speed. This last factor only affects flash exposure when ambient light exposure is more than flash exposure (caused by too slow a shutter speed) or if the shutter speed is so high that it exceeds the maximum flash sync speed (causing black bands in your photo).
There is one neat little trick that will help you estimate Flash Power quickly and fairly accurately. Flash Power is one of the factors which determine speedlight exposure. In order to help you determine the correct Flash Power, you can use your speedlight itself to give you a good indication. Here’s how:
Assuming that you’ve set up your scene by selecting all the exposure factors. You have chosen your ISO, aperture and shutter speed to create the right amount of ambient exposure, and now you need to determine Flash Power and Flash-to-Subject Distance. Pointing your Nikon or Canon speedlight at your subject, look at the LCD panel. In Manual Exposure mode on your speedlight, you will be able to see readouts of the Flash Power, Flash-to-Subject Distance, Aperture and ISO. For some reason, my Nikon speedlight requires its head to be tilted to a 90 degree position for this to work. Notice that Shutter Speed is missing from the readout, as it has the least impact on flash exposure (because it affects ambient light exposure).
Set the Aperture and ISO on your speedlight to match what you have on your camera, and set a temporary Flash Power setting (eg. 1/8). Now look at the Flash-to-Subject Distance readout. If it matches the distance between the speedlight you’re holding and the subject, your Flash Power setting is now correct. If not, adjust either the Flash Power, or move your speedlight closer to or farther away from the subject.
This works well for a bare speedlight, which means a speedlight without any light modifiers. But if you add a softbox or umbrella, you will need to compensate accordingly for the loss of light caused by the modifier.
Recently I’ve been experimenting with Gamilight light modifiers, which are really portable tools that fit into my existing bags. No extra bags to carry. The Gamilight Square 43 folds flat and fits nicely into my long bag which is used to transport light stands and tripods. In the wedding day portrait below, a Square 43 mounted onto my SB-900 speedlight was held by my 2nd shooter just out of camera view on the right.
If you’ve ever shot wedding day portraits, you know how little time we have to set up the shot and still get great results. The Square 43 is very light, takes less than a minute to assemble, and snaps onto my SB-900 speedlight with 4 buttons. A quick ambient light test shot confirmed the settings I needed to darken the background (which was quite busy, so I decided to darken it with the right combination of ISO, aperture and shutter speed). The light loss from this modifier is about 1 stop, which is manageable even with speedlights. I have also used the Square 43 as a hair light too, so it’s quite versatile.