Flash exposure is confusing for new photographers. Heck, it can be confusing for experienced photographers too. But I find that thinking about it in practical terms (by breaking it down into its component pieces) makes it easier to understand flash exposure. Once you get the physics of the flash, you can take it one step further and begin to control the flash and ambient light independently in a single exposure. Here’s how.
First, let’s look at the major difference between flash and ambient light. On a fundamental level, the big difference is in the duration of the illumination. While available light is constant—be it sunlight, room light or the flicker from a candle—available light is by definition available for an extended period of time. It’s the light’s constant nature that allows it to be modified by all three exposure controls: ISO, aperture and shutter speed. Adjusting any one of them adds or subtracts from the ambient exposure. But when it comes to strobes, only two of those controls matter.
Which one doesn’t affect strobe exposure? Well, it’s obvious when you stop and think about it. If my flash fires for just a fraction of the time my shutter is open, how could adjusting the shutter speed change the flash exposure? It can’t. Because if the shutter opens, and then the flash fires, and then the flash ends, and then the shutter closes, adding more time to the shutter speed won’t affect the flash because the flash duration is so much shorter than the shortest shutter speeds that traditionally sync with flash. It’s this shutter speed loophole that allows you to control flash and ambient light independently in the same frame.
For instance, let’s say you’ve got an on-camera flash set to half power, your ISO is 200, your shutter speed is 1/60thth and your aperture is ƒ/8. You could make the strobe appear brighter by, among other things, turning up the ISO one stop to 400. That will also, of course, make any ambient exposure in the scene get one stop brighter too. But then if you adjust the shutter speed from 1/60th down to 1/125th, you then darken the ambient exposure without impacting the flash at all. Remember, because it fires in just a fraction of the time the shutter is open going from 1/60 to 1/125 has no effect on flash, but it cuts a stop from the ambient—in this example, putting it back where it was before you adjusted the ISO.
To take this technique one step further, you can bring down the ambient even more by slowing the shutter speed to 1/250th. This is a great way to deepen a dark blue sky, or make a sunset more vibrant. It’s also useful for eliminating unwanted ambience that might be creeping in to the scene. If room lights are casting an orange tint on interior flash photos when you shoot at 1/30th, speed up the shutter speed to 1/125th and cut that orange cast by two full stops.
When you remember that you can also adjust a strobe exposure by dialing its power up and down, or by moving it closer or farther from the subject, you start to realize how many separate ways there are to control ambient light independent from the flash, giving you immense control over the kind of dynamic lighting that’s unique to flash photography.