An Introduction to Metering

Metering is the act of measuring the intensity of light in the scene you want to photograph in order to determine the “correct” exposure. Correct exposure is the exposure required to achieve the effect that you the photographer intend. It is determined by a number of factors, some of which you control, and others you have to live with: available light, which is a combination of ambient and flash light, film speed, shutter speed, and aperture. You can meter yourself, or let the camera do it for you. You can also guesstimate using the “Sunny 16 Rule.”

There are two approaches to metering: (1) incident, whereby you measure the intensity of light falling on your subject; and (2) reflected, whereby you measure the light bouncing back fromyour subject. Incident measurements are usually made using a dedicated, hand-held meter, while reflective metering can be accomplished using either a hand-held device, or by your camera’s built-in meter.

Reflected Metering

The meter with which most photographers are familiar is the one built into the camera, either somewhere on the front face, or inside the body. Because it points towards the subject, this meter measures reflected light. (Note that many older film cameras do not have built-in meters, and many others have broken meters, or meters that no longer give reliable readings. If you have one of these cameras, don’t despair: you may use a hand-held incident meter, described below, or shoot by the Sunny 16 Rule. Negative film is quite forgiving, and you should get acceptable photographs even if your exposure is not truly accurate.)

The first 35 mm SLR camera to offer “through-the-lens” (TTL) metering was the Pentax Spotmatic in 1964. The TTL meter measures the intensity of light reflected from the subject into the camera by way of the lens, which is the best way to meter from an in-camera perspective because this is the light that actually falls on the film. The Spotmatic needed to be stopped-down to the shooting aperture for a light reading to be taken, but more modern cameras do not suffer from this annoying limitation: both focussing and metering are done at maximum aperture, and the camera stops down only as the shutter release is pressed.

With the aid of an in-camera computer, the TTL meter has evolved to truly astonishing complexity, accuracy, and versatility. The fine detail is beyond the scope of this article, but suffice it to say that the modern TTL meter is typically capable of measuring the light in three basic modes: “spot,” “centre-weighted,” and “matrix.”

In spot metering mode, the camera accurately measures the light reflected from a very small patch (1-5%) of the overall scene, usually defined by the small circle at the centre of the viewfinder, without being affected by the surroundings. This is useful in high contrast situations where you need to ensure that the principal subject — a strongly back-lit face, for example — is properly exposed.

In centre-weighted mode, the camera will give you a light reading that is dominated by the central 60-80 % of the frame, which is where the primary subject is most likely to fall.

Matrix metering, also known as evaluative, honeycomb, or segment metering, was introduced with the Nikon FA in 1983 and has become the default metering mode of modern SLR cameras, both film and digital. It is deep magic. In principle, the best exposure is calculated by combining light measurements from several to over a thousand points in the scene. In making its determination, the camera takes many factors into account: autofocus point, distance to subject, in- and out-of-focus areas, colours, and backlighting. Exactly how this is accomplished is a closely held secret by camera manufacturers and varies from brand to brand and camera to camera.

In-camera TTL meters allow auto-exposure. In aperture-priority mode, you set the aperture, and the camera determines the appropriate shutter speed, based on a TTL light measurement, for correct exposure. Conversely, in shutter-priority mode, you select the shutter speed, and the camera determines the appropriate aperture. Many cameras allow you to deviate from the calculated exposure by dialling in an “exposure compensation.”

Incident Metering

In contrast to reflected light measurements, incident light readings are typically made using a hand-held meter. You make a reading by holding the meter right up to the subject so that its sensor points in the direction of the camera. In this way, you measure the intensity of the light falling on the subject from the camera’s point of view. If you’re not close enough to hold the sensor up to the subject, simply hold it above you with the sensor pointing backwards over your shoulder. As long as you and the subject are in the same light, everything will be fine. If not, you’ll have to guesstimate a compensation to what the meter gives you.

A handheld meter will give you combinations of f-stop and shutter speed that will render a well exposed photograph for a set film speed (ISO). You then choose from this list the combination that’s most appropriate for the effect you’re trying to achieve and dial it into the camera.

For example, the meter may give you 1/2000 s @ f/4, 1/1000 s @ f/5.6, 1/500 s @ f/8, 1/250 s @ f/16, and 1/125 @ f/22 as options. (Remember, the larger the f-number, the smaller the aperture and the less light enters the camera in any given window of time. Therefore, small apertures need slower shutter speeds to deliver the same exposure as larger apertures.)

If want a deep field (acceptable focus from near to far), you might choose 1/125 s @ f/22. Conversely, if you want a shallow depth of field, you might choose 1/2000 s at f/4. If you’re using a long focal length lens, say 200 mm, and are shooting a distant object, you probably don’t want to choose shutter speeds that are any slower than 1/500 s, especially if you’re not using a tripod. To minimize shake, you might therefore choose 1/1000 s @ f/8. You get the picture.

If you intentionally want to over or under expose, you’re free to select an f-stop & shutter combination that deviates from the ones your meter gives you. For example, you may want to bring out shadow detail in a scene by overexposing. Your meter gives 1/1000 s @ f/5.6 as an option, so you choose 1/500 s @ f/5.6 instead (at the risk of blowing out your highlights). Because the shutter is open twice as long, double the light will be allowed into the camera. This is overexposure by one “stop.”

Incident vs. Reflected

In-camera reflected-light meters are easy to use and, in more modern cameras, allow aperture- and shutter-priority auto-exposure, which makes grab shots faster and more reliable to make.

The primary advantage of an incident light meter is that it’s not affected by the colours in your scene. Light colours reflect light well, while dark colours absorb it. A reflected-light meter may therefore overexpose a dark subject and underexpose a bright one. As someone who’s shot a lot in Canada’s snowy North, I can personally verify that the second issue is a big one.