Metering

What is metering? Well, it's kind of a long story. Your camera is not all that clairvoyant when it comes to exposing a scene to your liking. The bad news? The camera tries to expose the scene, any scene, to a neutral gray. This is known as metering. The good news? You have some control over what the camera is "looking at" when it tries to meter the scene.

"Neutral Gray" may sound a bit strange when talking about color photography. Think of it as exposing to a medium density if the scene were evaluated in black & white. It is trying to find the middle point between the shadows and the highlights, and expose accordingly.

I will use this scene with extreme contrast to show the effects of each mode. The round portion of the white post is in the very center of the scene. This will matter on some of the modes. The scene has a bright white fence, a black mailbox, and the green leaves are near-netral. These ranges will show reasonably clearly what is happening in the exposure. The scene was shot in Aperature Priority set to 4.0 and ISO 100. The camera will calculate the shutter speed based upon the metering mode selected.

Your camera (at least Canon cameras) have four metering modes. These four modes can help you to expose a scene to how you envision it. The four modes are:

1. Evaluative Metering:

With evaluative metering, all the pixels in the entire frame are averaged, then exposed so that the average value is neutral gray. As you can see in the example, the scene is exposed pretty well for as "contrasty" as it is. The shutter speed was set to 1/250 sec. by the camera.

2. Center Weighted Average Metering:

This is similar to Evaluative Metering with a bit of priority on the center of the scene. This mode is probably the best for all-around shooting. The other three would be used for specific situations. In this case, the shutter speed was set to 1/200 sec. This indicates that the average of the pixels in the central area of the scene are slightly darker than the average of all the pixels in the scene. Thus, the slightly brighter exposure to compensate.

3. Partial Metering:

With partial metering, the exposure is based on the central 8 percent of the pixels in a round pattern. You can see that the round white area of the scene is driving the exposure. This photo is dark because it is trying to expose the white fence post to neutral gray. The camera set the shutter speed to 1/640 sec.

4. Spot Metering:


Finally, there is spot metering. It is nearly the same as partial metering, except that the "spot" is smaller. About 3.5 percent of the central area of the scene. Again, in a round pattern. In this case, it is likely that all of the white post fell into the "spot". The camera set the shutter speed to 1/800 sec.

Now that you know all this, how will you use it? Here is a real-life example: This is a nearly impossible scene to expose. It is an extremely dark foreground and a very bright background.


Using spot metering, the scene is extremely dark because the camera is exposing to the center (very bright) portion of the scene.

Here is a trick when spot metering: You can expose on one part of a scene, press the exposure lock button (it looks like an asterisk), then reframe the scene. For example, in this photo, I exposed for the darkest section which is near the front door. As you can see, it didn't work out well, but it does show the effect.

I then chose evaluative metering, then a bit of post-processing to end up with this final image: