Spot metering is my preferred metering mode on the Canon 50D. This allows me to nail the exposure first time -- 99% of the time -- by using AE-lock on my subject and applying the desired amount of exposure compensation. However, I find this too slow for some situations, so would like to learn to use one of the "full frame" metering modes (i.e. Evaluative or CWA).
I have tried using both of these modes. On the whole, I found Evaluative to be unpredictable. I found myself having to second-guess when and by how much the camera had automatically added exposure compensation. On the other hand, I found CWA to be much more predictable, but slower as it was still necessary to AE-lock and recompose for off-centre subjects.
It seems that Canon designed Evaluative metering largely as a replacement for CWA, and that by not using this, I would somehow be missing out on much of the ability of my camera.
How would I be better investing my time?
a) Using Evaluative metering and learning to recognise how it responds in certain situations so that I can second-guess it.
b) Using the much simpler CWA metering and learning to read the scene to determine the required amount of exposure compensation.
If only Canon had an "active focus point weighted average" mode, it would be perfect!
Answer
Metering will always be trial and error, because the camera assumes everything you're shooting reflects 18% of the incoming light back at the camera. It has no way of knowing whether your subject is white or grey, or even what part of your scene is the intended subject!
The closest the camera can get to knowing the latter is by looking at the currently selected focus point, as you suggest. In fact some Canon bodies do offer spot metering linked to the active focus point. I suspect it's only on the more expensive models. Edit: yes it seems to be just the 1D/1Ds models. And it's only activated when you're using a subset of the 45 AF points, which is disappointed (you'd think it could use the closest spot metering point when you have all 45 activated).
Contrary to ElendilTheTall I use Evaluative mode almost exclusively (no I don't use the camera on auto ;). The reason for this is I don't want to be constantly recomposing to meter. Also whilst Evaluative might not get it right as much as spot metering, but when spot metering gets it wrong, it can get it wrong by a huge margin. Example, if you happen to have a deep shadow in the centre of the scene you'll massively overexpose.
At the end of the day metering is fixable in post as long as you're close, unlike for example, focus. So when shooting quickly I concentrate on nailing the focus, even if it means the metering is off. Evaluative allows me to do this. If I'm not shooting quickly I'll bracket exposure or use manual to get the exposure exactly how I want it.
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