Start By Mastering Manual Exposure
There are enough variables in photography that, particularly once you add flash to the equation, manual exposure settings become a critical part of minimizing those variables and taking control over lighting. So the first step to mastering the mixing of flash with ambient is to truly master the manual control of the flash’s output.
Start by setting the flash to manual mode, usually indicated by a large M on the flash’s rear LCD. Then you’ll adjust the power up to full (often indicated as 1/1) or dial it down to half (1/2), quarter (1/4), one-eighth (1/8) and so on. Each halving of the output—from half to quarter power, for instance—cuts one stop of light. Increasing the distance of the flash to the subject also cuts light—and it, too, drops by two full stops with every doubling of distance. These two factors together begin to provide incredible control as you fine-tune the flash exposure. With any manual exposure setting dialed in (say, ƒ/8 at 1/125th at ISO 100) set your flash to half power and take a picture. Is it too bright or too dim? Adjust the power up or down by a stop as needed and try again. The beauty of the digital world is you can check your progress right on the back of the camera after each shot.
The other controls that you can use to adjust flash exposure are the ISO setting and the aperture of the lens. While shutter speed doesn’t impact flash exposure, ISO and aperture do. So if your flash exposure is too dark, for instance, you could up the ISO without changing any other settings and you’d see the flash exposure change. The only issue is, every other part of the exposure changes too—including the ambient light. So moving the flash and adjusting its power level are the easiest places to start when it comes to adjusting flash exposure independently of the ambient exposure.
Next, Get The Ambient Exposure Dialed In
With an understanding of manual exposure and flash controls in hand, it’s time to begin building the balanced lighting effect, and that starts with the ambient exposure. Just as you normally would, use a combination of ISO, shutter speed and aperture settings to dial in a manual exposure that looks pleasing to the eye. That could be over- or underexposed if you’d like to make a particular look (overexposed slightly for a high-key image, underexposed a bit if you’re looking for something more dramatic). In any case, determine the ambient exposure with the finished flash-balanced image in mind.
All things being equal, I suggest starting with a low ISO (such as 50 or 100) to minimize noise. Then choose a shutter speed that’s sure to sync with the flash—under 1/250th for sure (indicated by an X or red number on manual shutter speed dials), and likely more like 1/160th or 1/125th to ensure your camera’s shutter can synchronize perfectly with the flash. Lastly, and again if you’re not looking to minimize or maximize depth of field, choose an aperture in the middle of the range to maximize sharpness, provide a normal amount of depth of field and give yourself options to increase or decrease the exposure easily by adjusting the aperture up or down. Once you’re happy with the look of the ambient exposure, you’re ready to move on to step three.
Finally, Add The Flash And Take The Image To The Next Level
With the ambient exposure dialed in, you’re ready to add the flash. Let’s take a moment to consider why you might want to do this. In some cases, flat ambient-only lighting might be just a bit too boring. Look at the ambient example here. It’s not unflattering, but it’s nothing special and a little dark. If you want to control the illumination on the subject’s face, adding flash to the mix is the perfect approach. Plus, because you can control flash and ambience separately, you can give the scene more drama than an ambient-only exposure. I do this by slighting underexposing the available light before adding the flash.
Once you’re ready, turn on the flash and ensure it’s set to manual. With the flash set to, say, half power, take a picture and determine how the different elements are working. Is the flash on the subject too bright or too dark? Is the background too light or dim? You can control the flash and ambient separately, which is why this technique is so useful.
For a too-bright flash, consider dialing the power down a stop—from half power to quarter power. Alternatively, you can use the aperture to dim the flash. Changing from ƒ/8 to ƒ/11 will make the flash and the ambient a full stop darker. You can counteract the ambient change by then slowing the shutter speed from 1/125th to 1/60th, and now you’ve effectively altered the flash exposure without impacting the ambient.
If the flash is too dark, dial it up to full power. If that’s still not enough, move the flash closer to the subject or consider changing the zoom setting of the flash to make it a narrower beam that will effectively increase its intensity.
If neither of these adjustments provides enough flash exposure, again turn to the aperture or ISO. Opening up from ƒ/8 to ƒ/5.6 will brighten the flash by a stop, along with the ambient. That can be counteracted by changing the shutter speed—but only to the point that the camera will still sync with the flash. So you’re likely to do better dialing down the ISO one stop, and then slowing the shutter speed from 1/125th to 1/60th to again counteract the one-stop change to the ambient exposure.
If you want to take this technique to the next level, the first step is to get the flash off camera—maybe on a light stand or held by your outstretched arm just a couple of feet from the camera. This generally works wonders because on-axis light isn’t always flattering in the way a light deliberately positioned off camera can be. But even if you have to use your light attached to the camera’s hot-shoe, don’t fret: it can still look great if you take your time and control your exposure deliberately.
Another option is to add a gel to the flash to make it contrast dramatically with the background light. If your camera is set to daylight white balance, an orange-gelled flash will glow like a sunset. You can also then adjust the white balance to tungsten so that the orange-gelled flash will render as neutral white light, while the daylight background will shift more blue. In this way, gelling the flash a contrasting color is a great way to amplify the differences between the flash and ambient lighting mix.