If you don’t understand how your camera’s light meter works, you will likely be bewildered at its exposure choices when shooting in automatic or a semiautomatic mode and misled when shooting in manual mode. By learning about what the different metering modes on your camera do to determine correct exposure, you will learn how to take full control of your exposures.
This video from The Photographer Academy provides an easy explanation of how it all works and offers some tips on what to do to get your exposure right in-camera.
Cameras will typically have four metering modes – Evaluative (Canon) or Matrix (Nikon,) Partial Metering, Center Weighted Average, and Spot. To understand any of them, you must first understand that your camera has a specific goal in mind when it takes an exposure reading, and that goal may not align with your own in a given circumstance.
What a camera’s meter wants to do is expose for 18%, or middle, gray. This does not refer to the hue, it refers to the reflected level of brightness. If what you are photographing falls in the 18% gray territory of brightness, your camera will expose it correctly, or accurately instruct you how to expose it correctly if you are shooting in manual mode. For example, green grass is pretty close to 18% gray in brightness, so if you are shooting a scene with a lot of green grass, there’s a good chance your camera will get it right.
However, if you are shooting a scene that is intended to be very dark or very bright, your camera’s 18% gray goal will lead you astray. If your scene is very dark, the camera will want to overexpose it in order to achieve its beloved gray. Conversely, if it’s very light, the camera will be confused and want to darken it to be gray. With this knowledge, you can predict where your vision and your camera’s will diverge and take action to correct the issue.
As for the metering modes, each takes a different approach to determine the correct exposure. Evaluative or Matrix looks at the entire scene to attempt to average it all out to 18% gray. Center Weighted takes a similar approach, but it places significantly less importance on what is outside of the center of the frame for exposure determination. Spot metering looks at a very small portion of the frame around your focus point to determine exposure. Partial metering, which is generally a Canon mode, is similar to Spot metering but uses a bit more of the frame to make its decision.
The embedded video has some great, straightforward teaching in it so if you’re struggling with this and have just under 10 minutes to spare, have a watch for an “aha!” moment.