Kathy, if your lens is set to 'A' while it's on an extension tube, the diaphragm will be at its minimum aperture, probably f22. If you take it off 'A', when you press the shutter button, it will stop down to the aperture you set . Until you press the shutter, it will remain wide open, for easier viewing and focusing, just as it would when mounted directly on the body. This is the Auto feature.
To sum up, yes, the lens will use the aperture you set. The metering will be centre-weighted. I'm not sure whether spot-metering is available, but evaluative (multi-zone) is not. Now go take some pictures! Pat White
