Justin Serpico , Oct 22, 2007; 02:22 p.m.
Your camera measures reflectance off the scene.
In spot it measures only the center area.
In center it weights the center and then gives decreasing weights to the essentially cone shaped area outside of the center spot. this means you get an "average" also know as center weighted average.
In matrix, the camera essentially guesses based on evaluating the whole scene. via each segment. It's a more complex average but a lot of test have shown center weighted is as accurrate in most cases as 6 or 45 metering. Actually, even more test have shown above a few segments exposure accuracy changes very little.
Your shutter speed or aperture change in Av or Tv modes because you are moving the camera to different areas of reflectance.
Your cameras meter only sees shades of grey. It trys to put whatver you point it at as a mid tone exposure (18%ish) grey. If you point it at a white wall, your camera makes the wall grey because it assumes the wall is mid tone since it is the only shade in the scene.
If you point it at a black wall it makes the wall? you guessed it, GREY.
So if you had a 1/15th second exposure on a white wall and you moved the camera to a black wall in Av mode you'd get a 1/60th exposure and still a grey wall. Really the correct exposure for the wall should be 1/4 second on the white wall and 1/250th on the black wall for the wall to be either black or white.
Finally, when you link the AF point to the AE it actually just center weights based on that segment. So it's essentially non center, center weighted.
Personally, I spot meter most of the time. Either select a mid tone and meter that, then lock in the settings in manual. Or select a lighter and darker area and adjust that. Such as the black wall, and close down 2 stops which would make you meter show -2EV in the meter on a black object.
I use center weighted for backlit subjects in rapid shooting with a +1 Exposure comp dialed in. And for just fly by wire rapid shooting I set it to multi segment/matrix and figure the camera can do the rest.
Also, for very even lighting with low tonal range multi segment does just fine.
Oh, as far as metering the sky, it's at a 45* angle to the horizon, and you should generally open up about 1/2-1 stop because the sky isn't usually true mid tone. Of course usually it's close enough. But this assumes mid day sun with even lighting. In a high contrast scene you need to make the call as to what your mid tone is, and if that mid tone fits into the 6-7EV range of the sensor/film. That EV range is usually 4 under mid tone and 3 over mid tone.