What is exposure metering?

In photographic terms, exposure metering refers to how your camera measures the light in your image to determine the appropriate exposure. Since the camera doesn’t know what you’re pointing it at, it tries to balance the exposure to middle grey, commonly known as 18% grey. There’s a technical explanation of why it’s 18%, but really not worth getting into. This is one of those things that is best to just accept it as truth and move along. Kind of like the surface temperature of the Sun being 5,500 C. Somebody way smarter than me has figured these things out, so I just use the info for what I need and accept that I cannot know everything. Anyway, I digress. An exposure for 18% grey means your image will be properly exposed, ie. not over-exposed (too light) or under-exposed (too dark). Most recent cameras (digital and film) have a few metering modes you can choose from based on what you want to acheive and the lighting situation.

Common exposure metering modes

Let’s look at the 3 common exposure metering modes using a difficult photograph to expose correctly. In this case, the subject is backlit and the difference in contrast between foreground and background is more than the camera can record. In other words, the sensor (or film) is not able to record so many shades from very light to very dark, so it has to compromise. Each metering mode differs in how they decide on this compromise. For reference, all images were shot in Aperture Priority on a Panasonic LX3 and are straight from the camera.

• Evaluative / Matrix

This is the most common metering mode and usually the default setting in your camera, also called Pattern or Multiple. It divides the scene into several zones and measures the light in each of these. Then it does a bunch of calculations to determine the appropriate exposure. The number or zones and the calculations vary by camera and manufacturer. The important thing is that the camera takes into account the entire scene to come up with the combination of shutter speed, aperture and ISO that make up what it thinks is the correct exposure. Some cameras even compare the scene to exposure information in their database. The photograph below of a highly contrasty scene was taken using EVALUATIVE metering. The camera looked at the whole scene and figured there’s a lot of bright area (sky) and relatively little dark (subject), so it ends up with a compromise where the background is roughly well exposed but there’s not much detail in the subject.

Matrix Metering: 1/1000 @ f/5.6

Although in this particular situation it might not give a great result due to the high contrast between foreground and background, today’s cameras will perform pretty well for most situations using this mode. As long as there aren’t very large bright or dark areas, the exposure will usually be fine. I leave my cameras in this mode for probably 80% of the time.

• Center Weighted

Is kind of a combination of matrix and spot metering in that the camera takes into account the entire scene, but gives more importance to the centre of the image. It’s basically trying to expose the center correctly, but without ignoring the rest. In the example below, you can see it does an in-between job of the other modes by brightening up the subject while still trying not to blow the highlights in the background. In this particular image it ends up still underexposing the subject (although not as bad as with matrix metering) because the difference between the light and dark areas is just too much for the camera.

Center Weighted Metering: 1/640 @ f/5.6

I don’t use center weighted metering, if at all. I find it easier to just use Matrix and compensate manually the exposure with EV if necessary (another post on EV to come soon).

• Spot

In Spot metering, the camera measures the light from just a very small area in the center of the screen while ignoring everything else in the frame. The camera will suggest a combination of aperture, shutter speed, and ISO that will turn what’s in that middle area 18% grey, no matter what happens to the rest of the image. Note that some cameras allow you to move that “center spot” around so it doesn’t have to be in the center of the image. Take a look at the image below. The camera set the exposure for what’s in the center spot, exposing it for 18% grey. We see the subject well exposed with all the detail showing (now we can really tell what it is), but the background is blown out since the camera ignored everything outside of the center spot.

Spot Metering: 1/160 @ f/5.6

Spot metering is usually used in high contrast situations. For example, when the subject is backlit as in the photograph above, where the background is extremely light and the subject is too dark. Imagine shooting someone at the beach with the sun at their back. You can either expose for the background and get a nice sunset but the subjet in sillouette, or you can expose for the subject but get the background blown out (as in the example above). The opposite is also true, when the subject is light in a very dark background. Take a music concert for example. If the stage is illuminated by bright lights and the rest of the stadium is very dark, the camera will usually see too much dark and compensate by brightening up the whole thing. You’ll end up with pure white where the band would’ve been and properly exposed heads of the audience. But if you use spot metering and point it to the band, the camera will then expose them properly. You’ll then have a very nicely exposed band with everything around it mostly black. A much better picture, I think.

Hopefully this makes sense. There’s no better option as they all depend on what you’re photographing, what the lighting conditions are, and what you’re trying to do. For reference, I keep my camera in Matrix most of the time and only go to Spot when I feel the situation calls for it (like the concert example above). I never use Center Weghted, but that’s just me.