Spot metering

[neuroanatomist]

Why is spot metering limited to the center point only?

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.

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?

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.

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"?

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.

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?

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.

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

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.