Some Notes on Canon’s Evaluative Metering
I’ve been shooting with Canon SLRs for a long time, and for the most part, I’ve always been pleased with the camera’s metering. Granted, I can never remember which icon corresponds to which metering mode, but now that I keep the PDF of the manual on my phone, I can always look it up. During a recent shoot, though, I came across a curious detail about Evaluative metering that I never knew – one that can dramatically alter metering behavior in certain situations.
Consider the following:
I shot this image with a Canon 5D Mark III. Metering was set to Evaluative and focus was set to automatically select a focus point. The camera set focus on the black bowl, and metered at 1/40th of a second at f4.
But watch what happens if I manually select a focus point. In this image, I manually set a single focus point on the black bowl.
I’m still in Evaluative metering, but with my focus point manually set, the camera has metered entirely differently. It now says 1/30th of a second at f4.
Now watch what happens if I move the focus point to the white bowl:
Again, while I’m still in Evaluative metering, the camera has metered completely differently. It now comes in at 1/100th at f4.
I’ve always assumed that Evaluative metering was a straight matrix metering mode. That is, that it divides your scene into a grid, meters each cell of the grid, and comes up with an overall metering. In fact, it does do that, but if you manually select a focus point, then it gives more metering weight to the area around that point.
I usually keep my autofocus set to a single point in the center of the image, and I usually use Evaluative metering. I’ve been assuming all along that I was getting a matrix type metering, but what I’ve really been getting is more of a center-weight metering.
I probably should have been paying more attention to the name. If it was a straight Matrix metering mode, Canon probably would have named it as such. Instead, they called it Evaluative, and I assume that’s because it evaluates metering based on more than just its matrix analysis – it also includes focus point in its considerations.
Now, I’ll be honest, I don’t think this has really ever caused me a problem, but in a situation like the one shown above, your metering will get very screwy. I’m simply presenting this information here so you don’t get confused if you find yourself shooting a boring picture of black and white bowls.
One other note: when you select Spot metering, metering does not follow your focus point. Instead, Spot metering always reads the very center of the frame. This is something that possibly could get you in trouble if you tend to work a lot with the Canon Spot meter, so be careful.
Note that none of this is specific to the 5D Mark III. This is simply how Canon’s metering and focusing works on their SLRs.