Originally posted by Peter Zack I've experienced this under or over exposure issue with M series lenses as well. If you test as Lowell suggests and get used to how the lens reacts, it's not an issue. I believe it's because the K series cameras use a 'crippled' mount and just don't meter these lenses as accurately as an A series with the automatic meter coupling.
Peter, what I have found, and I can only comment on *istD and K10D (as I own them) is that there is a definite issue on the K10D with respect to metering when ANY lens is not in the A setting.
the problem I have seen, and I have described many times, is as follows>
at very large apatures, F1.4 for example, the K10D has a tendancy to under expose by 1 stop when metering using the green button, (or DPOF to activate stopped down metering).
As you stop the lens down, by about F5.6 this can move from under exposure to over exxposure of 1-2 stops, with worst over exposure at about f8-11 and then it begins to correct a little, as you get to F22 or F32.
My *istD does not show this, and I have even seen it with the K10D on my brand new tamron 28-75 F2.8, when I take the apature ring out of A.
What I suspect (obviously can't prove and pentax won't say) is that they have some sort of problem with the meter, the geometry, light reflection etc... that causes a definite error as a function of F stop largely independant of focal length. They have put a correction factor in for this,. and can compensate for AE lenses. BUT since on a K mount they don't know the maximum apature, they do not know how to correct for the error.
I don't know if it exists as a problem on any other body (as I said, my *istD is very good at metering with K mount lenses) and I would really love for people to do testing with each body and lens.
As you indicate, I have "mapped" my lenses, and know what they do, so I am a little prepared for this, and until pentax addresses the issue, that will have to do.