Spot metering

Why is spot metering limited to the center point only? When I had a 40D and used spot metering, it was no big deal as I was usually using the center point anyway. Now, not so much. If in Evaluative metering, it reads the whole scene reguardless of AF points, why can't it then read just from one AF point for spot? I'm not sure how this metering thing works I guess. Does it use a "big bucket" theory and doesn't really know whatcolor of light is coming from where, it just knows that once itfills the bucket, the final color is "X"?..or does it know all colors everywhere in which case there would be no reason for not being able to spot focus or docenter weighted on certain AF points? I'm also trying to get out of recomposing for this reason too. I could on the other hand have misunderstood the little manual and it actually does adjust to specific points and I've been working arouind it for no apparentreason.

In my XSi, evaluative metering, which links to the selected AF point, is almost a "spot metering" in the selected AF point; I mean, in my camera at least, evaluative metering doesn't seem to meter the whole scene right, as sometimes I get quite overexposed or underexposed areas (I don't know if other bodies have better evaluative metering).


If you use the back button for focusing, you can use spot metering, lock exposure and then recompose and focus.

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Originally Posted by andnowimbroke

Why is spot metering limited to the center point only?

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Because that's the way Canon designed it. The 1-series cameras have the ability to spot meter around several of the AF points, and can also take multiple spot meter reads and average them). Those features aren't available on non-pro bodies.

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Originally Posted by andnowimbroke

If in Evaluative metering, it reads the whole scene reguardless of AF points, why can't it then read just from one AF point for spot?

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Evaluative metering actually weights the metering to the area of the selected AF point(s). It's not true spot metering, but it does give more weight to the subject (as determined by the AF points) than the background.

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Originally Posted by andnowimbroke

I'm not sure how this metering thing works I guess. Does it use a "big bucket" theory and doesn't really know whatcolor of light is coming from where, it just knows that once itfills the bucket, the final color is "X"?

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Actually, it's mostly about intensity and less about color (although the 7D and T2i have iFCL metering, which also uses color information). With the 7D, evaluative metering measures the intensity and color of the 63 separate zones, weights the calculation according to the active AF point(s), and compares those data to stored 'scenes' to determine the proper metering.

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Originally Posted by andnowimbroke

or does it know all colors everywhere in which case there would be no reason for not being able to spot focus or docenter weighted on certain AF points?

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There's no technical reasonusing just one of the 63 zones over a selected AF point for spot meteringcouldn't be implemented. But, there isa reason it isn't implemented - that reason is called marketing. If you require that feature, Canon makes you buy a 1-series camera to get it.

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Originally Posted by andnowimbroke

I'm also trying to get out of recomposing for this reason too.

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As Carlos pointed out, that's why you'd use AE Lock (the * button). If you're using Spot Metering and want an off-center composition using an off-center AF point, you first point the central spot-metering circle at the subject, press AE lock, then recompose, focus and shoot. See Canon's article onUsing Auto Exposure (AE) Lock.

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Originally Posted by neuroanatomist

As Carlos pointed out, that's why you'd use AE Lock (the * button).

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That's what I have been doing. It just seemed like I shouldn't have to, but now that you tell me about the 1-series having it and the marketing around that, it all makes sense. Thanks again Carlos and Neuro for your time.


@neuro Congrats on your almost 1000 post!!!

You may be interested in this article as well: http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=2286. This is the way I use my XSi (shutter button to lock exposure and back button (*) to focus).

Thanks for the info, guys! I was driving myself nuts thinking there's a custom function that would spot meter on the focus point I'm using (40D, in this particular case).