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Poring Over the Picture

In most situations I simply rely on my camera’s exposure meter to determine my base exposure and adjust ISO, aperture, and shutter speed to capture the effect I desire. But when faced with lighting challenges, having a good understanding of light and my camera’s meter gives me the skill to make appropriate exposure decisions that result in images that represent my vision.

Exposure Defined

“What’s your exposure?” is a question I am asked frequently when in the field. What is exposure? Wikipedia defines exposure as the total amount of light allowed to fall on the photographic medium (sensor) during the process of taking a photograph. The three elements that play big roles in the making of well-exposed images are ISO, aperture, and shutter speed. When properly mixed and matched in various combinations, those three elements provide your image with the mood you are trying to convey. Today’s cameras are highly sophisticated machines. They can evaluate a scene and calculate what they consider to be a good exposure based on selected criteria that you have dialed into your camera, all in a split second. That criteria includes ISO, aperture, or shutter speed, as well as the focal length of your lens, and yes, even adding in the focal point used in the overall calculation. In even, mid-tone scenes (Figure 3.1) I rely on my camera’s built-in light meter to give me the proper exposure value. I select the appropriate ISO and aperture to suit my vision of the finished image and then let my camera match the correct shutter speed based on my settings and the light.

Figure 3.1 Diffused, even light allowed me to let my camera establish the correct exposure value for this Blue Grosbeak while I concentrated on the bird’s behavior.

Let’s take a look at ISO, aperture, and shutter speed—the three elements that make up the exposure triangle—their relationship to each other and to light, and how the various combinations of the three affect the end result of your images.

Exposure Triangle

For every correct exposure there are several exposure combinations that will result in the same exposure value but with very different results in the overall look of your images. Which combination you choose is directly related to the story you want to tell—from a sharp, storytelling image where everything is in focus (great depth of field) to one where the subject stands out in sharp relief against the softly blurred background (shallow depth of field), which is controlled by the aperture you select. When the light is low, the ISO you choose directly impacts the aperture and shutter speed combinations available; the trade-off for increased ISO may be greater noise in your images. And the shutter speed you end up with determines whether you will stop action (fast shutter speed) or blur motion (slow shutter speed). If any one element changes, the other settings will be affected. Using the exposure triangles in Figure 3.2 as a starting point, let’s look at the result of changing your settings.

Figure 3.2 A The exposure triangle with a basic exposure on a partly sunny day: ISO is set to 100, aperture is set to f8, and the resulting shutter speed is 1/250 sec.

B 100 ISO, f11 at 1/125 sec. Changing the aperture from f8 to f11 causes the shutter speed to slow from 1/250 sec to 1/125 sec.

C 200 ISO, f16 at 1/125 sec. If you need to maintain the shutter speed you have but still need more depth of field, increase the ISO to 200 and close down the aperture to f/16 while staying at 1/125 sec.

D 400 ISO, f16 at 1/250 sec. If you need to increase the shutter speed but still keep the depth of field, increase the ISO to 400 and leave the aperture at f/16, which increases the shutter speed to 1/250 sec.

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Note

Each of the exposure settings in Figure 3.2 is the equivalent of the same amount of light reaching the sensor. But the result can be dramatically different depending on the combination you choose in a given situation.

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ISO

ISO is the camera sensor’s sensitivity to light. The lower