Originally posted by kypfer Theoretically, this should work, but, in my experience, and probably only with some lenses and/or some cameras, there can be a tendency to over-expose.
I have never tested it out*, but believe your observations are traceable to incremental improvements to M mode stop-down metering starting with the K-7.** Whether such are applied with manual aperture lenses in Av mode is hard to say.
BTW...You may have have noticed that stop-down metering with your *ist D is spot-on. That is because its screen is old school.
Steve
* I could do it right now, but am too lazy.
** The theory here is a bit convoluted, but relates to the stock focus screen's brightness not being linear to the lens' physical aperture. This may be demonstrated by observing the viewfinder brightness while stopping down a manual aperture lens. With an f/2 lens mounted, brightness will be the same at f/2.0, f/2.8, and f/4.0 (even to f/5.6 with some subjects). Open-aperture metering (e.g. Av mode) allows the camera to compensate for the screen, but without knowing the set aperture, such compensation will not be possible. I don't know how corrections are applied for stop-down metering in M mode, but they appear to work pretty well. FWIW, I generally do stop-down metering in live view for greater accuracy.