In evaluative the camera reads the scene and pics an exposure. Many times this is fine, but particualry backlit scenes this will not work - so with spot metering you can point the camera so the 'spot' is on what you want to be properly exposed (a person's face for example) and then lock that exposure, move the camera to recompose the scene and shoot.
Canon is not big on this spot meter idea. My 300D will go to center weighted in M mode only. I can hardely wait to get teh 30D for this feature alone!
In M you select the aperture and shutter. Does it still need to meter?
The camera meters in all modes...in M (manual) you set the apeture and fstop based on the info given from metering. Manual mode allows you to make all corrections for conditions and make artistic control decisions.
My 300D will go to center weighted in M mode only. I can hardely wait to get teh 30D for this feature alone!
In M you select the aperture and shutter. Does it still need to meter?
It doesn't "need" to, no, but it will show you a little graphic in the bottom of the viewfinder and the needle will move as you change settings, so you can see where your chosen settings place the exposure on a scale of under-to-over.
Everyone needs to bear in mind that all reflective metering systems (which is what cameras like ours use) make the assumption that what they are pointed at should be rendered an average, zone 5 gray. In the case of an "averaging" metering mode, it's like the meter throws allt he values in a bucket, stirs them up to a uniform tone, and assumes this should be zone 5 gray, and gives you settings based on that assumption.
In spot metering, the camera assumes the spot is zone 5 gray.
So if you are metering a field of snow, for example, in spot OR average mode, you need to give more exposure than the camera calls for or your image will be underexposed. To put it another way; set your camera on auto and point it at a black wall that fills the frame and snap a shot; now go fill the frame with a white wall, and snap another. In theory, both will look the same, medium-gray.
"Evaluative" metering modes are a little more sophisticated, they use an algorithm that makes some assumptions about the scene based on light values from different portions of it. Some evaluative metering cameras will, in fact, give a darker image from the black wall than from the white wall. This is why I don't use them; I want to make these decisions myself.
If I have a lot of bright, I'll add a stop or two to the exposure; if I have a lot of dark, I'll subtract a stop or two. I mostly work in manual mode and just pay attention to that graphic in the viewfinder. But I also mostly shoot static subjects from a tripod, so that's a luxury I can afford.
You may not wish to expose correctly, i.e. achieve an exposure close to how a scene would present itself at first glance. You may wish, instead, to expose creatively, i.e. according to how you conjure the scene in your mind's eye.
You may even wish to send a whole background into oblivion, if the only data it holds consists of no more than a few distractions or redundancies, by deliberately over- or underexposing. I often use a spot meter to isolate a chosen subject (as opposed to the found one(s) etc...
