I understand mirror slap can introduce unwanted vibration. But what about the closing of the aperture ring, especially with a telephoto lens? I'll be using exposure delay mode with a tripod. When does the aperture ring close, when the mirror is raised or just before the shutter is released?
It can conceivably vary by camera model and lens model, but for the most part most SLRs with mirror lockup stop down the aperture at the same time the mirror is raised. I've never noticed one capable of mirror lockup that didn't.
Many of Nikon's older SLRs (and some of their not quite so old DSLRs) use the same mechanical lever to drive both the mirror and the aperture control linkage, so the action has to be performed at the same time. Even cameras/lenses that have electronic aperture control normally do it the same way. My Canon EOS cameras all stop down the electronically controlled aperture via the servo motor in the lens at the same time the mirror is raised when mirror lockup is enabled.