Quote:
Originally Posted by MOmilkman
This is what I know about metering. It evaluates the amount of light that is used in a particular frame.
Thats about it.
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Yup, that's about it. Metering measures the amount of light coming into the camera. The camera does calculations based on the iso/aperture/shutter speed settings to try and tell you if the exposure you're going to get is a good one or not, but it's kind of dumb in how it thinks and evaluates the scene.
Basically, all the values of light and dark in the scene are averaged (or weighted, or ignored/counted, depending on your metering mode). The problem is that the camera doesn't know what the scene is or what you want it to look like. It's just mechanically computing an average(ish) value and mindlessly setting that value to the middle of the exposure scale (middle gray).
If you have a good distribution of dark-to-light, the average really will be what you want to have set to middle gray, and the AE system will be very close to what you want.
But if you're shooting a night scene, the majority of your frame is going to be very dark, and you may want to keep it at black or close to black. But the algorithm is dumb, and the auto-exposure system will still be trying to set the average, a very dark/near-black value, to middle gray: upshot? you're likely to overexpose. Similarly, if you're taking a shot of a snow scene, the AE system will try to see a very light/near white value to middle gray and you're likely to underexpose.
This is why we like having exposure compensation and/or Manual mode to nudge the exposure one direction or another, based on the scene we're trying to capture.
One tool your camera offers you, in addition to metering, to let you judge exposure, is
the histogram. Using that together with metering can often help you get exactly where you want to be, exposure-wise, as the histogram lets you see where the light/dark values sit, related to your camera's dynamic range.