Capturing the Light: 3 Elements of Exposure

29575489331_60c9381cc6_oBy Caleb Rainey

Photography is the art of capturing light. The word “photograph” comes from the Greek words photos—“light”—and grapho—“to write.” Indeed, photography is in its most basic sense writing or painting with light. Thus, controlling the light you are “writing” with is a key element of making a great photograph. There are three main variables controlling the amount of light coming into your camera: shutter speed, aperture, and ISO or film speed. By understanding these elements, you can shape the light in your photographs in interesting and creative ways.

1. Shutter Speed

The first variable in your exposure is shutter speed. Shutter speed is the easiest of the three to understand—it’s simply a measurement of how long your camera’s shutter is open and letting light hit the sensor. It is usually measured in seconds or fractions of a second (1, 1/4, 1/16, 1/100, etc.). Older and higher end cameras have a physical shutter curtain, but smaller digital cameras often have a digital shutter instead. Here is a view of the shutter on my film SLR, closed (left) and open (right):

The principle remains the same regardless of shutter type—the longer the shutter is open, the more light is hitting your sensor. A good analogy for light in your photography is water. Think of your photograph as a bucket that you’re filling. If light is the water you’re filling it with, then the shutter speed is the length of time you have the hose on. The biggest effect that shutter speed has on the quality of your photographs is the capture of motion—if your camera or an object in the frame moves while the shutter is open, it will appear blurry or smeared in the photograph. This is not always a bad thing, but you will often want to avoid it. Thus, if you are photographing an object in motion and wish to capture it sharply, then you should use a faster shutter speed.

Here I use a slow shutter speed to capture the motion of playing the guitar.

Almost all cameras will let you control the shutter speed, but they are quite diverse in how, so it’s best to consult your camera’s manual to learn how to do so on your camera. Some cameras have a “Tv” or shutter priority mode, which allows you to set the shutter speed, and the camera will compensate for your changes by adjusting its aperture.

2. Aperture

The second variable in your exposure is aperture: the opening in the lens through which light passes. Aperture is expressed in f-stops, with lower numbers signifying the larger opening and larger numbers signifying the smaller opening. If this seems confusing, here’s what it looks like visually:

The picture on the left is the lens with the aperture set at its widest opening, f/1.8. The picture on the right is the aperture at a slightly narrower opening, f/4.

To continue the analogy of water, the aperture is the size of the hose’s opening. Thus, if your aperture is large, the bucket will fill quickly. If the aperture is small, then the bucket will fill slowly. This mean that aperture and shutter speed are inversely related: a smaller aperture requires the shutter to be open longer and vice versa.

Aperture affects your photograph in an important way: your depth of field. The depth of field is the area of your photograph in which objects are in focus. A larger aperture gives a narrow depth of field, and a smaller aperture gives you a wide depth of field. Depth of field is especially apparent when your focus point is close to the lens:

Narrow depth of field: the objects immediately in front of and behind my focus point are blurred because I am using a very wide aperture.
Wider depth of field: the background of the photo is much clearer because I am using a very small aperture.

As the focus point moves farther from the lens, the effect lessens. However, if you are shooting a landscape, setting your aperture smaller will produce a sharper image—a good benchmark is f/8. As with shutter speed, the method of controlling the aperture varies from camera to camera, and you should consult your manual. Also similarly to shutter speed, some cameras have an “Av” or aperture priority mode, which allows you to set the aperture and have the camera automatically set the shutter speed appropriately.

3. ISO or Film Speed

The final variable in your exposure is ISO or film speed. ISO controls the sensitivity of the sensor in your digital camera. It is measured in a system of numbers established by the International Standards Organization (thus the abbreviation). Most cameras begin at ISO 100, and go up to ISO 1600 or 3200, though newer cameras sometimes go much higher. Each doubling of the number represents a doubling in sensitivity—ISO 200 is twice as sensitive as ISO 100, and ISO 400 is twice as sensitive as ISO 200. This scale originated as a measure of the chemical sensitivity of a film to light. Before digital cameras, the only way to change ISO was to change the roll of film in your camera. Digital cameras, however, allow for the adjustment of ISO at will. To finish the water analogy, ISO is the bucket itself. A lower ISO is a larger bucket, since it is less sensitive, and a higher ISO is a smaller bucket, since it is more sensitive. If your aperture and shutter speed do not admit enough light, you won’t fill the bucket all the way and your photograph will be too dark. If they admit too much light, then the bucket overflows and your photograph is too bright.

Other than exposure, ISO affects the quality of your photograph by making it more or less grainy or”noisy.” Higher ISO numbers will make your photo grainy and less appealing:

The photo on the left was shot at ISO 100, and the photo on the right was shot at ISO 3200. As you can see in the magnified portion of the photo, ISO 3200 produces a much grainer image. The effect is more pronounced in low-light situations:


In this photo, I took advantage of the grainy quality of an image shot at high ISO in order to purposefully create a gritty, dramatic image. Often, though, you will want to avoid making your image grainier than necessary to capture the shot.

Knowing how to control your exposure to produce the effect that you wish opens up a world of possibilities, from the conventional to the bizarre. Shooting a sports event and want to freeze the action? Use a wider aperture and higher ISO to lower your shutter speed. Shooting a subject that you want to isolate in focus? Use a faster shutter speed and lower ISO to open your aperture wider and narrow your depth of field. In extreme or creative situations, you can use a very long shutter speed to produce some fascinating photographs:

In this picture, I use low light, a small aperture, and a low ISO to hold my shutter open for a long time and paint a “ghost” of myself on the church steps with a flashlight.
Here, I use a slower shutter speed and change the zoom of my lens while the shutter is open, creating a “zoom burst” effect where stationary objects appear to be in motion.
By using a long shutter speed and multiple flashes, I create several sets of hands playing the guitar without the use of photo editing software.

Now go forth and have fun with your camera! The more familiar you become with the inner workings of your camera, the better the art you will be able to create. In addition, you can submit the results of your efforts to our fall semester photography contest!

Caleb is the General Editor of SLAM and a senior at the College at Southeastern. He enjoys photography, music, a good bit of writing, and a hot cup of coffee. He hopes to use the arts to glorify God and to engage the culture for the sake of the Kingdom.

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