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How A Flash Is Like A Double-Exposure

Someone recently suggested to me a new way of thinking about flash exposures. This piqued my interest, of course, because among students and young photographers it appears that understanding the intricacies of flash, and in particular of balancing flash with ambient light, is quite a challenge. So in an effort to bring more understanding to the concept of mixing ambient light with flash exposures, allow me to offer a slightly different way of thinking about flash: every flash exposure is like a double exposure.

See, the concept of being able to control flash and ambient separately within the same exposure can be confounding. But when you realize that you’re essentially dealing with two different exposures in the same frame, it gets a little easier to comprehend.

Here’s the deal: with a normal ambient light exposure, your aperture opens narrow or wide to control the amount of light allowed into the camera. Then the shutter speed dictates whether the shutter is open for a very brief period or a much longer period—this is the duration of the exposure. Just as if you were filling up a bucket of water, both the amount and the duration combine to determine how efficiently the bucket gets full.

Armed with that knowledge, we can think about a flash exposure. This is the bit that seems to give so many trouble, but I think if you remember that an exposure is all about the amount of light allowed into the camera and the duration the light is allowed in, when you consider the fact that a strobe exposure is WAY FASTER than your shutter speed (which, let’s face it, is always going to be 1/250th or slower in order to properly sync with the flash) then you realize that adjusting the shutter speed (the duration the shutter is open) has no effect on the strobe exposure. The only way to change the strobe exposure (aside from altering the power of the flash or its distance from the subject) is to adjust the aperture—altering the amount of light allowed into the camera. That’s the rub: shutter speed has no affect on strobe exposure, but aperture does.

So, now that we understand making aperture changes affects both strobe and ambient exposures, and making shutter speed changes only affects the ambient, we’re free to harness this knowledge to control ambient and flash exposures independently in the same shot by thinking of the flash as a double exposure.

First let’s establish the ambient exposure. Let’s say it is 1/125th at f/11. If you adjust the shutter to 1/60th it will be overexposed, and at 1/250th it will be underexposed. Just like if you adjust the aperture to f/8 it will be overexposed, and at f/16 will be underexposed. So at 1/125th and f/11 you’re creating the first exposure in the frame, the correct ambient exposure. Then when the flash fires, it’s adding to this a second exposure. If the flash is powerful enough or positioned just right, it will balance with ambient exposure and fill in the shadows and give the image a bit of pop.

But what if the flash isn’t powerful enough? What if it’s, say, a stop underexposed? To adjust the exposure in camera you’ve got to adjust the aperture, from f/16 to f/11. But that will also impact the ambient, won’t it? So then to compensate for opening the aperture (letting in a greater amount of light) we need to shorten the shutter speed to 1/250th (and reduce the duration of the exposure). In this way, we can control the two exposures, flash and ambient, independently and simultaneously within the same shot.

If you have a hard time thinking about these two exposures occurring at the same time, reduce them to their simplest form—two separate exposures that act like a double exposure in a single frame.

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