OK, I admit I've never owned an FL camera and am only book-read on this, but I'm reasonably sure of what I've read :-)
Originally Posted by dynachrome
An FL camera does stopped-down metering. This means that the lightmeter in the camera is reading the actual light reaching it, which means that when turning the aperture ring the view through the viewfinder gets darker (the aperture is actually closing). This is as opposed to the later FD wide-open metering, where turning the aperture ring does not close the aperture, but rather moves a lever on the back of the lens that tells the camera's meter what the aperture is going to be when the picture is taken, and it does the electrical "math" required to take that into account versus the light reaching the meter.
If you put an FD lens on an FL camera in its normal mode, then no matter what you do with the aperture ring the aperture is wide open, and the meter has no way to read the lever on the back of the lens, so there is no way to meter (unless you are planning to shoot wide-open.) The only way to successfully stop-down meter with an FD lens is to put it in FL mode so that turning the aperture ring actually closes the aperture as you do it. The "A" mode has nothing to do with it - that's for the later shutter-priority-auto-exposure cameras and finders.
My understanding of FL lenses (having not used one, as I said) is that the "M" mode is for metering (moving aperture ring closes aperture) and the "A" mode is for composing and taking the picture (lens stays wide open until the moment of exposure, when the camera stops it down to the aperture set on the aperture ring.) Of course maybe I have that backwards? This would be one advantage of a real FL lens over an FD lens in FL mode: with the FD lens in FL mode once you are done metering, your lens is still stopped down to wherever the aperture ring is set and you have to compose and focus through the darkened view. (The equivalent of the FL lens' A-M switch is the click-stopped lever on the back of the lens - not quite as convenient to operate while shooting!)