Understanding Exposure Controls

My tremendous high school photography instructor, Mr. Colgan, ground into our heads that cameras are deceptively simple, nothing more than a light-tight box with three modifiers. But how we use the three modifiers of shutter speed, aperture and ISO is where things get a bit more complicated and where all the technical magic begins to happen. For photographers who want to take control over their technique, learning to manually control these exposure modifiers is essential. To that end, here’s a look at the three controls that form the exposure triangle, as well as what they do, how they work together and how to prioritize camera settings based on what visual effect you’re trying to accomplish.

ISO

ISO is the setting that adjusts the camera sensor’s sensitivity to light. (Technically, the sensor has a default sensitivity and we’re just telling it at what level to output the information it gathers, but that’s getting a bit pedantic.) In practice, the lower the ISO number, the lower the sensitivity—and the stronger the signal-to-noise ratio will be. Low ISOs are 50, 100 and even 200; it’s best to start here by default and only raise the ISO as necessary. A higher ISO setting tells the camera to output the image at a brighter level, amplifying every bit of information it has gathered—both signal and noise. That’s why images shot at high ISOs, such as 6,400 and 12,800, appear to have lower quality and fidelity; there’s simply more “noise” interfering with the image-forming detail.

Shutter Speed

The camera’s shutter is like a curtain that opens up to allow light in via the “window” of the lens. The shutter speed is how quickly that curtain is opened and closed—the duration of the exposure. A fast shutter speed of 1/500th, 1/1000th or more is very brief and especially useful for freezing a fast-moving subject—an animal in nature, perhaps, or an athlete running and jumping.

Slower shutter speeds—anything below the 1/60th “default” for handholding a normal lens—will produce blurry photos when the camera is handheld or when the subject is moving quickly. That blur could be used for deliberate creative effect or it could ruin the shot—it’s all in the eye of the beholder.

To photograph a runner and impart a sense of speed, a deliberate motion blur could show their pumping legs and arms in blurry motion. For this, a slow shutter speed would be necessary, and trial and error is the best approach based on the individual subject’s specific speed. A fast shutter speed freezing that same athlete can also impart a sense of speed by allowing us to see sharp details that the naked eye could never discern. The photographer simply chooses whatever aesthetic she desires when setting her shutter speed for a given exposure. 

Aperture

The aperture, also called ƒ-stop or iris, controls the amount of light the lens allows into the camera. Continuing the window analogy from above, whereas the shutter speed represents how quickly a curtain on a window is raised and lowered, the aperture represents how large or small that window might be. As the photographic exposure is simply a gathering of light, you can see how a shorter duration and a smaller opening would allow less light in and make for darker exposures, while a larger opening (bigger window) and longer duration (slower shutter speed) combine for brighter exposures by allowing in more light.

Small apertures (such as ƒ/16 and ƒ/22) are tiny little openings represented by big ƒ-numbers. They produce greater depth of field—the area of sharpness from front to back through a scene—although at the extreme they also introduce diffraction that makes images slightly less sharp than apertures in the middle of the range. Wide apertures (such as ƒ/2 and ƒ/2.8) are bigger openings represented by smaller numbers. They make for shallow depth of field—a great technique for isolating a subject against an out-of-focus background, perfect for many subjects and especially for striking portraits.

In conjunction with the duration as controlled by the shutter speed, and the sensitivity as modified by the ISO, the aperture can be used to amplify or counteract those other settings—as they can be used to counteract aperture. This is the case with each of these exposure controls; all of them prioritize distinct photographic effects and all of them have consequences. For instance, a photographer who wants to prioritize shallow depth of field in order to better isolate a subject from a busy background will choose a wider aperture. But that wider aperture will also dictate a lower ISO (typically a good thing because of its lower noise) as well as a faster shutter speed. That could be a problem if you’re trying to balance with a flash or you want to introduce a bit of motion blur.

Prioritizing a slow shutter speed in order to introduce motion blur, however, may also necessitate stopping down the aperture to a higher ƒ-number—like ƒ/16 or ƒ/22. Depending on the amount of light falling on a scene, this could require a slower shutter speed—which could make handholding problematic—or dialing up the ISO to compensate and therefore decreasing the image quality by increasing the noise.

Whatever exposure needs the photographer prioritizes, the three exposure controls work together to balance the exposure and ensure it’s neither too light or too dark, too sharp or too soft. If you’ve determined that the appropriate exposure for that aforementioned running athlete is 1/125th sec. at ƒ/8 and ISO 100, you may decide you want to ensure their motion is tack sharp, so you increase the shutter speed three stops to 1/1000th of a second. That would necessitate compensation with one of the other two exposure controls—ISO or aperture—to prevent a three-stop underexposure. Do you increase the ISO three stops to 800 or open the aperture three stops to ƒ/2.8? I’d choose the latter most times, but then I’d risk missing focus due to the shallower depth of field. See? There’s always a tradeoff. And it’s determining which tradeoffs to make, what visual effects to prioritize, that makes one a photographer. Therein lies the art of the craft.

For photographers who are just learning to make sense of these disparate controls, don’t worry: we understand that it can be confusing. We’ve all been there! There’s a useful tool called the “exposure triangle” that plots each of these controls onto its own side of a triangle and makes it easier to visualize how to create equivalent exposures when adjusting one of the three camera controls. Opening up the aperture two stops? You’ll have to speed up the shutter speed two stops, or maybe just one stop plus one stop of lower ISO, in order to achieve the same level of exposure while prioritizing the effects that are most important to you.

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