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Camera Basics: The Three Ways To Adjust Manual Exposures—01/09/12

Understand these simple camera controls to become a manual-exposure master

This Article Features Photo Zoom

Does accurate exposure control matter any more? Since the advent of the TTL auto exposure mode, not to mention digital cameras with RAW image capture, does the right exposure even matter these days? The obvious answer is yes, because accurate exposures are the technical foundation for every great photograph. But what exactly are your options when it comes to manual exposure control? Turns out there are three: ISO, aperture and shutter speed. Keep reading for an overview of adjusting exposures using these three controls that have formed the basis of accurate exposures for generations.

The first digital camera setting that impacts exposure is something I like to think of as the footing upon which an exposure is built. It's the ISO setting, which is a measure of the digital camera's sensitivity to light. In the old days this was called "film speed," and the higher the speed, the more sensitive the film was to light. Unfortunately, high-speed films also featured prominent grain. Thankfully, today's cameras can deliver unbelievably high ISO settings with a very low noise profile. (Noise being the digital version of film grain—it increases at higher ISOs.) So how do you know which ISO setting to use? My starting point is always to use the lowest ISO setting that will provide the other exposure settings that meet my needs. That means a hand-holdable shutter speed (like 1/60th or faster with a normal lens) at an aperture that provides the depth of field I want. The lowest ISO ensures the lowest noise in the final image file. Noise-reduction technologies are amazing, and ISO 2000 in a modern DSLR is comparable to ISO 400 film in terms of the "noise," so things are vastly improved. Still, the lower the ISO setting, the lower the noise. This usually means choosing an ISO starting place of 100. You'll adjust this ISO if you need a higher shutter speed, or if the light gets lower and you need a larger aperture.

The next step in creating an accurate exposure involves a decision: Do you adjust aperture at this point, or shutter speed? If the shutter speed is your priority for a photograph (say you're shooting something fast moving, or you want to create a long exposure for motion blur) the shutter speed will be your next adjustment. If, however, your concern is primarily based on depth of field—whether you want a lot or a little—you'll be better served by choosing the ƒ/stop next. Let's say that's the case so you're going to adjust your aperture. How do you know where to go? Well, you consider depth of field. For a shallow depth of field—in which foreground and background objects should be out of focus, either for compositional simplicity or stylistic reasons—you'll want to choose the largest aperture possible. That is the fƒstop correlating to the smallest number, like ƒ/2.8 or ƒ/4. If, on the other hand, you want to maximize depth of field to keep foreground, subject and background as sharp as possible, you want to stop down all the way to the smallest aperture number, likely ƒ/22 or ƒ/32. Of course, doing this might require an adjustment to the third exposure control, shutter speed.

The final exposure control on your camera is the shutter speed. A shorter (or faster) shutter speed is great for stopping action—freezing a runner in mid-stride, for instance. Whereas a longer, slower shutter speed is great for introducing motion blur—say, for panning with a runner to create a blurry image that's less sharp but imparts a feeling of motion. There are many reasons why you might choose a slower or faster shutter speed, but the basis for all of them is sharpness vs. blur. Shorter shutter speeds make sharper images if you're hand holding the camera or if any part of the scene is moving, and longer shutter speeds introduce blur by way of camera shake (if you're hand holding) or motion blur (if your camera is steadied on a tripod but part of the scene is moving—like a runner or a waterfall or a wind-blown tree).

Each of these exposure controls affects the other two, which is why it's up to the photographer to prioritize what's important in his photograph. If, for instance, you've set the ISO on 100 and chosen a small aperture (like fƒ22) to maximize depth of field, but you also want a shutter speed of at least 1/60th or 1/100th to handhold your camera… Well, what if that combination creates a significant underexposure of one stop? You've got to decide which setting to adjust in order to create a usable exposure. This means going back to settings you've already established and changing your selection. You can always boost the ISO a stop (to 200, in this case) in order to maintain the current aperture and shutter speed combo. Or you can open up a stop to lose some depth of field but without sacrificing it entirely. Or maybe you can put your camera on a tripod and lengthen the shutter speed so you don't have to worry about camera shake at all. Or you can take option number four and make a small adjustment to each of these settings.

That's the nature of making an accurate exposure: it's all about prioritizing the settings that achieve the effects you want, and then understanding how best to compromise on those settings when they don't jive with the reality of the light. Which, of course, raises a fourth option for exposure control—adding or subtracting light. That's why professional photographers so often work with additional hot lights or strobes, so that they can add enough light to a scene so as not to compromise on any of their ideal camera settings. But that's for another day…

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