Originally posted by Marc Sabatella ??? that term applies to autofocus. Not relevant to mirror lenses. What you might be referring to is a discrepancy between the amount of DOF you see in the viewfinder as compared to in the actual shot; this is an unfortunate fact of life with most modern focus screen designs. Usually an issue only at very alrge apertures, but I could imagine it also showing up with a 500/6.3 because the DOF is probably at least as thin with faster "normal" lenses.
If the auto-focus is back-focusing, so ALSO will be the focus- confirmation dot; it is the same mechanism. You are of course correct that a thin DOF really highlights any backfocusing issue (which was my point); it was also super apparent with my manual pentax 50mm f1.4 at large apertures. I've adjusted the back focus through the firmware and am pretty happy with the results. Having the focus confirmation spot on makes this lens a lot more useful.
Quote: I'd call it "sort of" true - I can get a focus confirmation with my 500/8 about as often as I'd expect (which is to say, often enough to be sort of useful, if less often that I would with other lenses).
If I manually stop down another lens to f8, my split focusing screens pretty much black out and the focus confirmation gets real sketchy without good light. I'm saying that with the f6.3 mirror, both the focus confirmation and the split screens work very reliably (provided the camera is not back-focusing to begin with). I think the ability to actually focus in a variety of lighting situations is a reasonable trade-off for a narrow depth-of field.
BTW, I could shim the split-focus screen on my k10d a bit as it back-focuses slightly compared to the focus-confirmation dot which is spot on but this, again is something I only really notice with this lens. It is simply a matter of practice to remember how to line up the split image. (ie, catch the focus from the front and not from the back)
Quote: From what I understand, it's a property of mirror lenses in general, to deal with the possibility of thermal expansion/contraction or something like that.
Ahh, interesting. I think I'll add some strips of tape and mark where infinity is. Mostly, so I can set it at infinity quickly but it would also be interesting to see if it varies appreciably.