Pepe
the real difference is that the DOF preview is just that, DOF preview. with one slight exception, the meter is activated and if you look at the +/- metering scale, you will see how many stops over / under you are, but the camera leaves the setting alone. This is useful, because it gives you a sanity check, without changing your settings. I use DOF preview, specifically when using manual lenses with flash on my *istD, (which supports TTL metering) so I can check what contribution the ambient lighting will offer in blended flash modes. The same is true when using "AUTO" flash with an AF540FGZ on newer cameras, and legacy lenses because you can again see the ambient reading.
the green button, on the other hand, will set the exposure to what the metering thinks it should be, (as you say, not perhaps what you want, but generally a balanced exposure). It will attempt to set the P line for all lenses that report focal length back to the camera, i.e. all AF lenses.
on MF lenses, it will set the shutter to what ever is needed to set the exposure
I sometimes wish, that when a flash is connected, the green button would only activate the meter as long as you held it, but that is just me.
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