Excerpted from Chapter 5 of Concert Photography.
Determining the correct exposure is critical for producing the best possible negative or transparency, a goal that should always be foremost in your mind. Color and black & white negatives offer wide latitude for under- and overexposure that can be corrected somewhat during processing and printing, but image quality will suffer in either case. Underexposure causes a loss of shadow detail and produces a print that lacks a full range of grays or colors. An overexposed negative is always less sharp and shows too much grain because of light scattering in excessively dense areas. Still, it is better to err on the side of overexposure when shooting negative film because of the built-in latitude. Color transparencies are particularly unforgiving, however, allowing little more than a 1/2-stop error in either direction unless processing compensation is made. The price for that compensation is increased graininess and reduced color intensity. Your best ally in the quest for consistently accurate exposures is a good light (or exposure) meter.
It doesn't require a degree in quantum physics to understand the principles of exposure, although the confusing array of exposure modes and metering options on today's electronic cameras might suggest otherwise. There are only two elements involved-lens aperture and shutter speed. The aperture, or lens opening, controls the volume of light entering, while the shutter speed regulates the length of time that light strikes the film. Aperture size is measured in f/stops and determines depth of field, the distance behind and in front of the focus point that appears sharp (see Chapter 6 for more on depth of field). The amount of light is doubled each time the aperture is opened up (made larger) one f/stop. Paradoxically, the size of the aperture opening is in inverse relationship to the size of the number; i.e., f/22 is smaller than f/2.0. The smaller the f/stop (the larger the number), the greater the depth of field.
Shutter speed regulates the action-stopping ability of the camera and is measured in fractions of a second. The faster the shutter speed, the greater the action-freezing power. The same relationship between f/stops and light gathering applies to the shutter speed; slow it down one click (say, from 1/125th sec to 1/60th sec) and twice the amount of light strikes the film. Conversely, a faster shutter speed or smaller f/stop halves the amount of light with each change.
Because of this precise mathematical relationship, the same exposure can be supplied by a variety of aperture/shutter-speed combinations. For instance, the following combinations all permit the same amount of light to reach the film: 1/30th sec at f/16; 1/60th sec at f/11; 1/125th sec at f/8; 1/250th sec at f/5.6; 1/500th sec at f/4; and 1/1000th sec at f/2.8. Your task, usually accomplished with the aid of a light meter, is to find the aperture/shutter-speed combination that produces the precise amount of action-stopping power and depth of field necessary to capture the photo as envisioned.
The first step in making that choice is to decide which is more important for the conditions-aperture or shutter speed. If the action is particularly fast, then shutter speed will probably be paramount. If, however, you require more depth of field, then aperture will prevail. In concert photography, though, poor lighting conditions often narrow your options considerably. I prefer to start with a shutter speed of 1/125th sec and let the aperture fall where it will. When you drop much below 1/125th sec, especially with telephoto lenses, you're courting unsharp photos from either camera shake or subject movement. If your aperture is wide open and you still don't have enough light, then you have no choice but to slow your shutter speed. In that case, steady hands, an inactive subject, or some kind of support are your best allies.
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