Do you still use some of the legacy lenses on your Pentax DSLR(s) and carry out focusing manually and metering stopped-down? If yes, it would be always interesting to know if actually it is accurate or not for the aid which facilitates both, i.e., the focusing screen.
Here is an interesting recent article by an Olympus DSLR guy who has presented the case very clearly:-
Manual Focus with AF DSLRs
The article is self-explanatory on the problem found. Indeed, the focusing screen of his Olympus DSLR is not good for Manual Focusing (MF) with fast lenses. From my experience with Pentax DSLRs, I found that my *ist D and DS are quite reliable for MF for *any* lenses, including my very fast short tele-prime FA* 85/1.4 which often fooled me with incorrect focus with their AF.
However, for MF with my K100D, I found that it is not so reliable as my *ist Dx bodies. Maybe it is just the limitation of its viewfinder with a smaller magnification of 0.85X (versus the 0.95X of my *ist Dx), but it maybe also because of the design of the focusing screen is not good for MF with fast lenses, just like the Olympus case illustrated above.
It is known that the focusing screen of the K100D and K10D are newly designed, to make them even brighter, especially for that the K10D's one has been widely recognised to have metering problems with legacy lenses for stopped-down metering, which causes obvious overexposure. The more one stopped down, the more overexposure will result. Here are some of the related (numerous) discussions on the issue:-
http://forums.dpreview.com/forums/read.asp?forum=1036&message=21822089
http://forums.dpreview.com/forums/read.asp?forum=1036&message=22056769
As we follow down the threads, various people do suggest that by replacing the original K10D focusing screen with an *ist DS one, which is of the same size but of a different optical characteristic, can resolve the problem mostly, which makes the K10D to expose correctly regardless of the aperture selected. By doing so, one would usually need to swap back the K10D screen to the *ist DS. But ironically, this would again correct the underexposure problem of the stop-down metering of the *ist DS (and *ist D as well), which it was quite firstly reported by me, in the following full "measurbation" report in my homepage long time ago:-
http://www.geocities.com/ricehigh/FAStar_85_1_4.html
For the exact evidence on this method really works, see this systematic presentation with all the posted pictures, metered and shot with different f-stops:-
http://forum.digitalfotonetz.de/viewtopic.php?t=31356&start=21
(Post in German, use BabelFish if required)
So, for a Pentax DSLR user who has both the *ist DS and the K10D whom uses his legacy Pentax lenses on his DSLRs, he can get both cameras for the right exposure. This is what a secret and special gift which Pentax gives us! In all other cases, the exposure will be off, either overexposed or underexposed!
As for the K100D, my limited experience with pre-A lenses on it for stop-down metering is just right for the metering/exposure accuracy.
So, next time when we believe firmly that the MF with the focusing screen through the finder and metering stopped down TTL-ly (Through-The-Lens-ly), then we should think twice if this is really the case or not. The only reason for the errors, if any seen, is simply that the focusing screen is not WYSIWYG (What-You-See-Is-What-You-Get), it simply lies most of the time afterall. In fact, it is either not designed to do particular jobs and/or off-calibrated actually, especially for the metering inaccuracy case.
Read also:-
Focus Calibrations for (Pentax) (D)SLR Bodies and Lenses