‘Setting the exposure’ is the process of adjusting the camera controls so that the right amount of light will fall on the film when taking the picture.

So what is the ‘right amount’ of light? There is only one practical way to define this: it is the amount of light that gives the result you want in your picture. If you get the exposure wrong, you will judge the result (at least) either too dark in the shadows, or too light in the bright parts, called the highlights. It will not give the effect you were expecting.

There are three variables we can control in order to get the exposure right in any given photographic situation:

1. Film speed (or ISO setting on a digital camera);

2. Lens aperture;

3. Shutter speed.

Film speed is a numerical measure of the film sensitivity. Modern cameras, films and light-meters use the ISO (International Standards Organization) system of numbering, which runs as follows (intermediate numbers also exist, but are less common):

Slower films and lower ISO settings on digital cameras generally provide the best image quality. With increasing ISO, film images become subject to increased graininess, and digital images suffer from increased noise, which looks somewhat similar. Older light-meters may show film speeds in ASA (American Standards Association) numbers, which are effectively the same as ISO numbers, as shown in the table above. You may also still find DIN (Deutsches Institut für Normung – German Standards Institute) numbers. These have also been incorporated in the new ISO standard, but are not often used. It is a system in which the number increases by three for each stop of sensitivity, rather than doubling like the ISO standard, so the numbers are quite different.

On digital cameras you can regard the ISO setting as an exposure control variable. On film cameras, once you have loaded the film, it is essentially a constant that you’re stuck with. That leaves the aperture and shutter speed to decide on. You can increase exposure – make your image brighter – by opening the aperture (smaller f/numbers) or by using a longer shutter speed (smaller numbers again for most of the range, in which the marked values are the denominators of fractions of a second … 1/60, 1/125, 1/250, 1/500 … etc., marked as … 60, 125, 250, 500 … etc.). You can reduce exposure – make your image darker – by closing the aperture or using a shorter (faster) shutter speed (or both, of course).

In nature, the range of light intensity that we encounter is huge: it is not unusual for the brightest parts of our subject to be several hundred times brighter than the darkest parts. For this reason, the magnitude of the adjustments we may need to make is also fairly coarse: it has become conventional to measure these changes in ‘stops’. One stop represents a doubling or halving, depending on the direction of adjustment, of the amount of light that reaches the film. The name derives from the click-stops that are still found on the aperture rings on the lenses of some types of cameras. Many modern cameras provide for adjustments in 1/2 or 1/3 stop intervals, for fine-tuning exposure.

There are two fundamental tools involved in setting exposure: a light-meter and your brain. There are two basic types of light-meter: reflected and incident light meters. They are used differently but give you essentially the same information: the exposure required for an ‘average scene’. Your brain is the tool you use to decide if the scene before you is ‘average’ and whether or not you wish to portray it in an ‘average’ way.

Modern cameras generally have built-in exposure meters. These are reflected light-meters and we shall consider these first. With this sort of meter, you point it at the subject from the direction in which you plan to take the picture, and it will tell you what the exposure ought to be – based on the average brightness of what the meter has ‘seen’ and the ISO (film speed) setting. In terms of tonality alone, ignoring color for the moment, this means that the average brightness of the scene will be rendered as middle grey if you blindly follow the light-meter’s measurement. This may be what you want, or it may not.

Let’s consider two examples of situations in which you might be wise not to obey your light-meter. For a street scene, at night, there will likely be a few small sources of artificial light, with perhaps a shaft of light falling on your primary point of interest, and some reflected light from the wet pavement. Maybe 80% of the scene, however, is very dark, virtually black. The meter gives you an average reading so that the overall tonality will be medium grey, so to get the result you expect you should reduce the measured exposure so that the areas you want to be black really are black, and not a dirty grey. By how much should you reduce the exposure from what the meter indicated? This depends on the scene – probably one to two stops. At the opposite extreme, for a brilliant snowy ski slope, with here and there an isolated tree or a few distant skiers, the exposure measured by your meter will give you medium grey snow. You will have to increase exposure by about a couple of stops in order to represent the original scene as you saw it in real life. Backlighting is another situation in which you may have to increase exposure so as to prevent your subject from being too dark.

You can get a medium grey card and take your reading from that: place it under the same lighting as your primary subject. Then if your subject is lighter it will look lighter, and if it is darker it will look darker, which is roughly what you want. You can achieve this same result by using an incident light-meter, except that with this sort of exposure meter you hold the meter in front of your subject, pointing in the direction of the camera. It measures the amount of light incident on your subject. Outdoors, if your subject is far away, so that you cannot easily place the meter at the position of the subject, try to ensure that the light falling on your light-meter is similar in direction and intensity to the light falling on your subject.

Even though a reflected light measurement from a grey card and an incident light measurement should give the same result, and this result should relate to the scene in a rational manner, your exposure measurement may still not be exactly what you want. You may choose to make your picture lighter or darker, increasing or decreasing exposure, in order to control the mood of your image. All of these adjustments in which you change the exposure from what you measured with your light-meter require thought and experience, which is where your brain is needed.

Further information on exposure measurement, including the zone system, different film types and digital cameras, flash and other lighting situations will be found in the article Advanced exposure.