Metering modes

© 2009 Simon Hucko

Most cameras today come with the option of using one of three metering modes: a matrix (Nikon) or evaluative (Canon) mode, a center-weighted mode, and a spot metering mode. For this discussion I'm going to refer to matrix metering instead of evaluative since I shoot Nikon, but the principle is the same for all systems.

Spot metering is the simplest. Your camera meters from a very small spot, generally centered in the selected focus point, and sets exposure for that spot to come out at 18% luminance. 18% luminance is also called "middle gray," and is the standard for metering dating back to the advent of the zone system. This mode works well if you are metering off of a gray card, but if you're metering on anything else you're going to have to really understand tonal relationships (ie zone system) to get good, consistent results. Too much work, in my opinion, so I never use spot metering while I'm shooting.

Center-weighted metering is a little more complicated. In this mode, the meter looks at the entire scene, but gives 75% weight to a small circle in the middle when calculating exposure. It's similar to spot metering, in that the camera exposes mainly for the center target, but it's a little more forgiving and makes some attempt to avoid over- or under-exposure of the rest of the scene. I like to use this mode when shooting people indoors with my 50mm. I set up my camera to lock the exposure on the half-press of the shutter. When I go to take a picture, I put the middle AF area on the person's eye and focus/lock exposure, then re-compose the shot and take the picture. This way I get a nice consistent exposure on people's faces, independent of whatever's going on in the background.

Matrix metering is partially responsible for the high cost of your camera. The meter looks at the entire scene, and then a computer analyzes the tonal range, contrast, location of bright and dark spots, etc. It then decides on what type of scene you are metering and sets the proper exposure. Most of the time, it does a darn good job. I use this for my walk around shooting and rarely have to fiddle with exposure compensation. From what I've read, the Nikon metering system is designed to preserve highlight detail if possible, whereas Canon is less conservative and can lead to blown highlights if you're not careful. There are merits to both approaches, but that's a post for another day.

Knowing how your meter works and how it responds to different situations can help you get more consistent results from your camera, and save you a lot of time and frustration in the field and in post. As always, the best way to learn is to practice.
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Thanks to my friend Matt for the suggestion for today's post. If you have any questions or anything you'd like covered, feel free to ask (in comments or by e-mail or on twitter or whatever) and I'll see what I can do.

~S