Light metering

All of you must have seen these icons in the camera menu. But what exactly do they mean and why and when is it important to select one or the other?

Modern cameras meter light with very high precision and based on these measurements they determine exposure parameters – aperture, speed and ISO. Light can be metered in several ways. The basic ones are – evaluative metering (matrix metering – the first icon), partial metering only in certain areas of the frame (second icon) and spot metering (third icon).

Evaluative metering usually works in the majority of cases. The camera receives information from different parts of the frame and levels the exposure by trying to achieve balance between bright and dark areas. This type of metering is best for photos where there is no object to be defined as more important than the rest. For example, in landscape photography – everything is equally important – the lake, the peaks, the clouds, the grass, etc.

F-9, ISO – 100, S-1/200 sec, Multi segment metering mode

The frame you see above is not perfect, but even where there are complex light conditions, sharp contrasts and a dynamic scene, light meter performance is at its best. Light and dark areas are well balanced. If you would like to have more light in the shadow, then you will most certainly lose the good level of detail in the light areas and they will burn.

Matrix metering does not perform well when we have one important object in the frame which is very different in terms of brightness or colour from the other elements in the photo. For example – a portrait of a black man on a light background. The camera will automatically decide to level out light and dark areas and the face will become unrecognisable. There is no way the camera can understand that this small area of the photo is important for us without any further pointers.

F-5.6, ISO – 1600, S-1/1250 sec, Multi segment metering mode

From a technical point of view the photo above is correctly exposed – you can see the histogram in the lower right corner. Neither lights, nor shadows predominate. Balance has been achieved, we have sharp clear-cut detail of the shirt of the performer. From a human standpoint, however, this photo is void of meaning, because we cannot see the face of the performer. He is unrecognisable. So what if we can see the folds and wrinkles of his white shirt? They are not important for the viewer.

In such situations we must resort to the segment or spot metering function of the camera. By selecting one of the two modes, we are instructing the light meter and guiding it right to the area or spot which is important to us and which we want correctly exposed, while the rest of the photo is irrelevant. And the light meter starts operating in a different mode – it considers only the brightness and colours at the specific zone or spot, which is usually related to the focus area or spot.

In this way, even with strong back light, also known as contre-jour lighting, we can have a correctly exposed portrait. That is a correctly exposed portrait from an aesthetic point of view – being able to see the face. From a technical point of view, as you can see from the histogram, the photo below is overexposed with predominant light tones and a burned sky in the background.

F-2, ISO – 100, S-1/800 sec, Spot metering mode

When we use the contre-jour technique, it is critically important to choose the right metering mode. Our choice depends on what effect we want to achieve with our shot. We might want to have the typical contre-jour photograph, where the objects appear only as silhouettes and the focus is on the backlight, or we might want to focus on the objects, disregarding the background.

The next two shots were taken at the same time, with the only difference being the metering mode. On the left we have a photograph with evaluative metering and on the right – with spot metering, where the spot was selected on the clothes.

Sometimes the wrong metering, a defect in the picture, can turn out to be its greatest effect. The next photo was taken on film. Film cameras had only one type of light metering – evaluative (matrix) metering. Also, you did not have the opportunity to see the result, not before developing and copying the film, anyway. In this particular case, I trusted the light meter without taking into consideration the fact that the light caught by the water will confuse the matrix metering system and will limit the light. The result was that this photo, taken at noon on the quay in Sozopol, looks like night photography. But this I saw a month later, after I had copied the photos.

What seems like ages ago, in the times before cameras had in-built light meter, on the inside of the carton package of photographic film rolls you could find printed the following scheme, which photographers used as a guideline how to set the correct exposure time.

One should really use the camera as though tomorrow you’d be stricken blind.
Dorothea Lange