Understanding Exposure


Reproduced by kind permission of Chelmsford Camera Club.

Exposure is the most fundamental concept in photography, but it's a mystery to many folks. It's a mystery worth clearing up, though, because taking control of your exposures is the first step to making better pictures.

The word exposure has several senses in photography, but we'll stick with the simplest. Exposure is the amount of light that strikes your camera's film or digital sensor. It determines the brightness of your picture. Too much light entering causes pictures that are washed out, or overexposed; too little light causes shots that are dark, or underexposed. Your camera has to regulate exposure because brightly lit subjects reflect more light into your lens than do dimly lit subjects. (For the same reason, the iris of your eye widens and narrows to regulate exposure for your vision.)

A modern camera's built-in light meter, with all of its mysterious calculations, exists solely to figure out the so-called "correct" exposure-one that gives the most true-to-life rendition of your subject's actual colour and tone. Getting your exposure right is critical in all photography. You can use an image editor like Photoshop, or a darkroom enlarger, to correct a exposure that's modestly off the mark, but you'll get crummy results trying to correct a truly bad exposure.

ISO, Aperture, and Shutter Speed


Leaving aside flash photography for the moment, three camera settings determine exposure.

The first is the light sensitivity of your camera's sensor, expressed as its ISO setting.

In some all-in-one digital cameras the ISO is fixed-usually at 100-but it's adjustable in many other cameras. Higher ISO numbers equal greater light sensitivity. In other words, less light is required to produce the same brightness in the final picture. The ability to change ISO from shot to shot is one of digital photography's great advantages, but, as you probably guessed, there's no free lunch: as a general rule, turning up your camera's ISO degrades your picture quality, sometimes dramatically. Using a film camera effectively fixes your ISO number for the whole film.

The second factor that affects exposure is lens aperture, or "f-stop".

(Technically, aperture and f-stop are not synonymous, but most photographers talk as if they are.) The f-stop can be thought of as the size of the opening in the lens through which light passes; in most decent cameras it's adjustable (by you, or by the camera's exposure computer, or both.) You've probably seen f-stop numbers-f/2.8, f/4, f/8, and the like. Smaller numbers represent bigger openings, so f/4 lets more light through the lens than f/8, giving you a brighter picture. (If this numbering system seems counter-intuitive, it may help to know that f-stops are actually fractions-i.e. f/4 is really one-fourth and f/8 is one-eighth.)

Shutter speed is the third factor.

Shutter speed can be thought of as the duration that light is allowed to accumulate on the sensor.

In general photography, typical shutter speeds are fractions of a second, one one-hundred-and-twenty-fifth, for instance, and, of course, longer durations produce brighter exposures and vice-versa. But remember - longer durations show moving subjects as blurred images.

Whether you make your own exposure settings or let your camera make them automatically, all three of these values, ISO, Aperture, and Shutter Speed-combine to produce the exposure in every picture you take. For the sake of simplicity, we'll ignore ISO for the moment and look at how aperture and shutter speed interact.

For any given amount of light coming from your subject, only one combination of aperture and shutter speed produces the technically best exposure. Imagine that combination is f/8 at one two-hundred-fiftieth of a second for a particular outdoor picture. If you change to a wider aperture-f/5.6, for instance, without changing the shutter speed, you will let more light onto the sensor and thus overexpose your picture. But you can change to a wider aperture without overexposing.

How? By also changing your shutter speed-in this case to a faster value, one five-hundredth-that cuts the duration that light hits the sensor. In other words, the combination of f/8 at one two-hundred-fiftieth gives the same exposure as the combination of f/5.6 at one five-hundredth (or f4 at one-thousandth and so on.) This complementary relationship between aperture and shutter speed is photography's fundamental equation.

Because their numeric markings are different, it's helpful to think of aperture, shutter speed, and ISO settings as "exposure values" (EV).

If you change your aperture from f/8 to f/5.6, you have opened the lens (and brightened the picture) by one EV. A shutter speed of one five-hundredth is one EV shorter (and darker) than one two-hundred-fiftieth. ISO 200 is one EV more sensitive than ISO 100. Your camera's exposure compensation function is marked in EV units, too. If you set it to +1 EV, you've accomplished the same thing as opening your lens one f-stop. (Professional photographers often call exposure values "stops", from the term "f-stop", even when they're not talking about lens openings.)So why should anyone care about f-stops and shutter speeds-especially since most cameras can set them automatically? Well, for one thing, your camera will sometimes set the wrong exposure. This is why the exposure compensation feature exists, and why you need to know about exposure even if your camera won't let you make exposure settings manually.

Equally important, the combination of f/8 at one two-hundred-fiftieth may produce the same exposure as f/5.6 at one five-hundredth, but it doesn't produce the same picture. That's because aperture and shutter speed settings affect other very important qualities in your photos: aperture has a big influence on depth of field (the range of focus from near to far in a scene), and shutter speed determines whether motion is recorded as blurry or sharp.

In short, if you can understand and control aperture and shutter speed, you have two powerful creative tools at your disposal.