Exposure is a complex beast. Mastering it is of the utmost importance. Exposure and composition are the two most critical components towards making a great image. Exposure is based on three components:

The ISO or sensitivity to light
Aperture or the size of the opening that will let in light, and
Shutter Speed defines the duration the light will have to pass through the aperture

Whether you shoot in Manual, Aperture Priority, or Shutter Priority; there is no difference in how the meter evaluates the scene.

Measuring the light, or brightness of the scene you are trying to capture is a critical component in determining the perfect exposure. To establish this you need a meter that can read the levels of brightness.

Exposure is measured using light meters. There are two types: one measures the light falling on the subject or scene, called an incident light meter; the other measures the light reflected off the scene or your subject, and is called a reflected light meter. All meters that are built into digital cameras are reflected light meters, and we will be addressing that type in this article. The more you understand these meters and how they work, the better you will be at understanding and interpreting what they are telling you. Keep in mind that incident light meters are far more accurate than reflected light meters.

Every modern DSLR has something called “Metering Mode”, also known as “Camera Metering”, “Exposure Metering” or simply “Metering”. Knowing how metering works and what each of the metering modes does is important in photography because it helps photographers control their exposure with minimum effort and take better pictures in unusual lighting situations.

What is Metering?

Metering is how your camera determines what the correct shutter speed and aperture should be, depending on the amount of light that goes into the camera and the ISO. Back in the old days of photography, cameras were not equipped with a light “meter”, which is a sensor that measures the amount and intensity of light. Photographers had to use hand-held light meters to determine the optimal exposure. Obviously, because the work was shot on film, they could not preview or see the results immediately, which is why they religiously relied on those light meters.

Today, every DSLR has an integrated light meter that automatically measures the reflected light and determines the optimal exposure. The most common metering modes in digital cameras today are:

Matrix Metering (Nikon), also known as Evaluative Metering (Canon)
Center-weighted Metering
Spot Metering

Some Canon EOS models also offer “Partial Metering”, which is similar to Spot Metering, except the covered area is larger (approximately 8% of the viewfinder area near the center vs 3.5% in Spot Metering).

A camera meter is not only useful for just the Manual Mode – when you choose another mode such as Aperture Priority, Shutter Priority or Program Mode, the camera automatically adjusts the settings based on what it reads from the meter.

When To Use Partial And Spot Metering

Partial and spot metering give the photographer far more control over the exposure than any of the other settings, but this also means that these are more difficult to use — at least initially. They are useful when there is a relatively small object within your scene which you either need to be perfectly exposed, or know will provide the closest match to middle gray.

One of the most common applications of partial metering is a portrait of someone who is back-lit. Metering off their face can help avoid an exposure that makes the subject appear as an under-exposed silhouette against the bright background. On the other hand, care should be taken as the shade of a person's skin may lead to inaccurate exposure if this shade is far from neutral gray reflectance (although not by as much as with backlighting).

Spot metering is used less often because its metering area is very small and thus quite specific. This can be an advantage when you are unsure of your subject's reflectance and have a specially designed gray card (or other small object) to meter off of.

Spot and partial metering are also quite useful for creative exposures, and when ambient lighting is unusual.

Notes On Center-Weighted Metering

Center-weighted metering was once a very common default setting in cameras because it coped well with a bright sky above a darker landscape. Nowadays, it has more or less been surpassed in flexibility by evaluative and matrix, and in specificity by partial and spot metering. On the other hand, the results produced by center-weighted metering are very predictable, whereas matrix and evaluative metering modes have complicated algorithms which are harder to predict. For this reason some still prefer to use center-weighted as the default metering mode.

Which metering mode is best?

When creating an image, I find the best option is always to shoot in Manual mode and set the exposure yourself by controlling the aperture, shutter speed, and ISO.

This is how I work, whether I’m shooting in the studio or on location. I usually set the aperture first and then set the shutter speed based on that. I only use the camera’s light meter to get me in the ballpark, but I never know what metering mode I’m actually using. I make my exposure decisions based on the results I see and the histogram information.

If you’re unsure of which settings you need, using your camera’s metering system can be helpful, especially if you know which metering mode to use for a given scenario.

Evaluative metering, as I’ve already briefly touched on, is a good option to go with if you are shooting scenes where you want to get the exposure as close to correct as possible for the entire scene. It’s also a good option if you’re unsure of which mode to use.

Landscape images are one example where you may want to use evaluative metering. Using this mode will help you get the best overall exposure for the whole scene, even if there are bright highlights or deep shadows.

Centre-weighted metering is best suited for any image where your subject is in the centre of the frame, or even where the subject fills the majority of the scene. A typical example of this would be portrait photography or macro photography. The key thing to keep in mind when using this mode is that the rest of the scene may well be under or overexposed.

For any scene where you want to expose for a very particular area, spot metering would be the best choice. Examples of where you might use this would be when photographing the moon or any high-contrast scene where you want the subject to be correctly exposed.