I just read this really helpful journal by Garry Kirsch http://garrykirsch.deviantart.com about metering. He kindly let me share it on her.
“Spot, Centre Weighted, or Evaluative Metering
Which is best?
It’s a trick question. The answer is “whichever one is best for the type of photograph you want to take” – and every type of photography is different.
Here’s a quick run-down on the differences:
Evaluative or Matrix metering (for landscapes and such)
This type of metering divides the scene up into zones, then analyzes each zone for highlight and shadow. It then takes an average for all the zones and determines the exposure based on that figure.
Center-weighted metering (for portraits)
Center-weighted metering gives the most importance to the light that is concentrated in a circular area in the center of the frame (like a person’s face). The corners are given much less importance, though they are usually included to a small degree in the calculation.
Spot metering (for small subjects – like birds )
Spot metering measures the light in a small part of the scene – usually less than 5% of the total scene – and usually in the centre.
Spot metering is a very accurate form of metering in that it will give you a precise reading for a very small part of the scene, so it is most useful for shooting high-contrast scenes where your subject may otherwise fall into shadow or be washed out by very bright highlights – like a bird on a branch with a bright sky behind it.