Last night I was working with some students on lighting for photography, and I pulled out my Minolta IIIF flashmeter and checked the output of light. I read off the settings to a student, she adjusted her camera to match, and took a photo.
Normally, that's kind of impressive -- it's an accurate meter, and usually that first shot is pretty close to perfect. But her results were overexposed.
Well, I thought, maybe there's a calibration issue here. So she adjusted, and adjusted, and still, her shots were over.
I didn't realize it until later, but it's very likely that -- since she was using an older lens on her new camera -- the issue is that there is some broken down oil in the lens that is stopping the lens from closing down quickly enough to the desired aperture.
In other words: a lens sits with its aperture on its widest setting while you look through and frame your shot. When you press the shutter, it very quickly stops down to the aperture you've set, then opens back up to fully open after the shot. (That way, you are seeing the brightest view when you look through your viewfinder, but the shot is taken at the aperture you select.)
But older lenses that have been stored sometimes have the oil inside break down -- and this can mean the aperture sticks a lot or a little, causing just the kind of problem we were experiencing. Changing her aperture setting didn't seem to change her exposure -- meaning when she was set on f/16 the lens couldn't close down to f/16 and it may have really been shooting at f/5.6.