So far three camera controls have been discussed separately: focus, aperture, and shutter. Focus is the most straightforward because it is used to produce a sharp image of the subject. Aperture and shutter each affect the image in two distinct ways. They both control the amount of light that makes the exposure, and they both affect image sharpness. The aperture alters depth of field, and the shutter controls the image movement or blur.

The light-sensitive material must receive the correct amount of light to produce a quality photograph. Under most lighting conditions, it does not matter whether you use a wide aperture with a fast shutter speed or a small aperture with a slow shutter speed. When the combination is correct, both provide the same amount of exposure.

Aperture and shutter speeds each have a doubling and halving effect on exposure. This doubling and halving relationship of aperture and shutter allows you to combine different f/stops and shutter speeds to alter the image, while, at the same time, admitting the same amount of exposure to the light-sensitive material; for example, you have determined that the correct camera settings for your subject is 1/125 second, at f/16. Instead of using this combination of shutter speed and f/stop, you could double the shutter speed (to stop action) and halve the f/stop. In this example your new camera setting could be 1/250 second at f/11, 1/500 second at f/8, or 1/1000 second at f/5.6, and so on. Or when you need more depth of field, 1/60 second at f/22 or 1/30 second at f/32, and so on, can be used. These shutter speed and f/stop combinations are called equivalent exposures. Equivalent exposures are used to control depth of field and to stop motion. Table 4-3 shows some equivalent exposures of a typical situation.

Each of the combinations in table 4-3 produces the same exposure; however, the amount of depth of field and image blur are different in each image. The combination of shutter speed and f/stop is used to best capture the subject and effect you want to create.

You should use a light meter for most of the photographs you take. The light meter provides you with a number of f/stop and shutter speed combinations; however; depending on the situation, the level of light alone can determine the camera settings. For example,

Table 4-2. Action Stopping Shutter Speeds for Normal-Focal-Length Lenses

Table 4-3. Equivalent Exposures

the light level may be so low that you have to use a slow shutter speed and the largest f/stop to get the proper exposure. After determining the correct exposure, you can decide how to present the subject. Remember, depth of field can be used to emphasize your subject, and shutter speed affects subject blur.