In the last two issues of the magazine, our basic digital photography tips have focused on how to use shutter speed and the aperture to manipulate what the viewer sees in the final image, but we also noted that these settings are used to decide how dark or light the picture will be.

However, neither the aperture nor the shutter speed alone is responsible for how the photograph is recorded. One more factor plays a part. This is the light sensitivity of the film or digital sensor. Digital cameras have a setting which allows the user to adjust an ISO number. ISO is a measure of how sensitive the recording medium (either the film or the digital camera’s sensor) will be to light. The higher the number, the more rapidly it will react to and therefore record light flowing through the lens. Conversely, the lower the number, the less sensitive the sensor, and therefore, the more light needed to reach it before an accurate picture is recorded.

The tradeoff at high ISO settings, where only a small amount of light is required by the recording medium is that a level of disturbance is introduced into the image. For film the effect is called grain, and gives the appearance of randomly distributed dots which make up the picture. For digital, it is called noise and it can often be more uniform and give an appearance not unlike a picture printed in a newspaper. While film grain has been used to great visual effect by many photographers, the noise recorded by high ISO settings in digital cameras is almost universally derided as visually unattractive.

The ISO is indicated by a set of numbers, which for most films and digital cameras, double with each increment in the setting. It would normally run 100; 200; 400; 800; 1600. This is to indicate that with each increment, half the amount of light is needed to achieve the same effect.

Now bare in mind that, traditionally, the aperture and shutter speed are also incremented by a doubling in size or speed, and it should be clear that a change in one of these settings needs an equal change of one of the other in the opposite direction in order to record the same amount of light and for the final image to be equally dark or bright.

Think of the three as a triangle. When one of the corners are moved, either one or both of the other corners will have to move too, if the same amount of light is to be recorded.

For example, say we start with the camera’s ISO set at 400, the shutter speed at 1/250th of a second and the aperture at f16. Say we decide we want less depth of field, meaning we will need to adjust the aperture, but we don’t want the picture to become too light. We must then change the size of the aperture to f11, which doubles the size, and doubles the amount of light which passes through the lens, and to make sure that too much light does not cause the image to be too light, we must either halve the amount of time the shutter remains open and move it to 1/500th of a second or halve the sensitivity of the recording medium and change the ISO to 200 for an accurate exposure.

What is an accurate exposure?

While the ISO setting is normally adjusted by the user, most (dare I say all) digital cameras have a light meter built in, and can adjust the aperture of shutter speed for the user in the instant that the photograph is taken.

But even though there have been tremendous advances in the way that these meters measure how much light is recorded, the machine will always be fallible because it cannot anticipate the effect that the user wants to create.

For most, average scenes, the camera’s meter will do just fine and will give a well exposed photograph, but the moment it is presented with a tricky set of circumstances, or the photographer is attempting an unusual visual effect, the trouble starts.

Sometimes the camera will be tricked into recording too much light, which will result in an image which is too dark. This is called overexposure. Underexposure is when not enough light was recorded, and the image is too light.

The classic example is when photographing a snow-covered landscape. The camera’s meter ‘looks’ at the scene and measures a lot of light bouncing off the snow. However, it has no way of knowing that the scene is almost pure white, and calculates that it should darken the image down to compensate for the large amount of light which is hitting its sensors. The resulting photograph will show the snow much darker than it should be, almost to the point where it will appear grey. In this case the photographer will have to compensate for this error in measurement, either by overriding the meter completely or by setting the exposure compensation dial to negative one or two stops (-1 or-2).

Another situation where the camera often fails is in very high contrast situations or when there is a light source in the frame, such as when you are taking a picture of an outdoor Christmas tree after dark. The overall scene is very dark and would need a long exposure at a large aperture and high ISO setting, but the camera is often fooled by the small areas of bright light (in this case the Christmas lights) into thinking that it is looking at a very bright scene. It is not uncommon for the camera to underexpose by as much as 3 or 4 stops, which could result in very disappointing images. In our case, none of the details in the tree and branches would be discernable, all one would be able to see is the specs of bright light in a see of darkness. To compensate for this the photographer will have to either override the meter completely, or adjust the exposure compensation dial to +2 or +3.

But the automatic light meter’s fallibility lies not only in its tendency to be deceived, it is also in its lack of creativity.

There may very well be cases where the photographer would want to underexpose a scene to create the visual effect he is striving for. A silhouette is but one example of underexposure working to the advantage of the visual impact. Another example of the usage of underexposure, where the whole image is recorded darker than normal, creating a brooding, ominous feel to the shot, is called ‘low-key.’ The opposite, where overexposure is used is called ‘high-key’, This can be used to great effect, such as when you are photographing a lonesome figure walking in a desert. Especially when photographed in black and white it would give the idea of intense heat radiating off the landscape.

In such circumstances, and many others, it would be wise to treat the light meter as your slave, not your master. Don’t be afraid of it, adjust it and expose, see what happens, then change it and try again. The visual possibilities are endless.