It’s the actual brightness of the ambient light that will dictate the correct exposure, of course. The point being that simply creating a correct flash exposure and then slowing the shutter speed until the ambient light registers is one way to balance flash with ambience. This “brute force” approach definitely works, but I prefer approaching the problem from the other direction by starting with the ideal ambient exposure.
Instead of working backward from a correct strobe exposure, why not start with the perfect ambient exposure? That doesn’t necessarily mean you need the subject to be brightly illuminated by the ambience; after all, if you’re working in low light, the overall scene probably still should be fairly dark. But this approach allows you to create the ideal ambient exposure for your scene. Maybe it’s simply a twilight sky filled with color—and then add flash to the subject in order to create the perfect balance.
The main advantage to this approach is that you can boost the ISO enough to keep the shutter speed fast enough and the aperture wide enough to still have a hand-holdable exposure. The higher the ISO and the smaller the ƒ-stop, the lower you’ll need to set the flash power. That’s why I prefer this approach—a little bit of flash fill goes a very long way.
Let’s say you’re making a portrait at sunset, and let’s say after a bit of trial and error, you reach an ideal exposure for the ambient background of ISO 800, 1/30 sec. at ƒ/4. Now all you need to do is add your flash to illuminate the subject. With the flash in auto-TTL mode, it will add a fairly low-power flash—which you could dial in manually by starting with your flash’s lowest output and then adding power, as needed. If you can’t get the flash to a low enough setting, simply move it farther from the subject. If, for some reason, you can’t get enough power out of it, simply move it closer.
If you’re feeling overwhelmed by the details in this technique, remember that it all boils down to one key element: that flash exposure isn’t affected by changes to the shutter speed, but the ambient exposure is. Balancing the two exposures in the same shot is a simple function of understanding your camera controls and putting them to work deliberately.
Lastly, bear in mind that the more ambient light is falling directly on the subject, the trickier it will be to separate the subject from the background. Ideally, the ambient light will be isolated to the background, and the subject is essentially a silhouette. This way, the longer shutter speeds needed to up the ambient value won’t also up the ambient light on the subject; they will remain in shadow until you fill them in with a flash. Also, if you choose a setting in which there’s some light in the background (like a sunset sky or the lights on Las Vegas Boulevard), it can be much easier to practice the principles—both because it takes less of an exposure to create an appropriate background and because it can be much easier to position your subject in a silhouetted situation.
Originally published July 18, 2014. Updated November 28, 2018.