I have a couple of observations here for anyone caught in the decision to go with older manual focus and or manual aperture lenses.
first of all, the K10D and K20D are known for very poor metering using the standard focusing screen.
I have posted the following before but it is useful to post here also.
I have a *istD, K10D and K7, the K10D I have checked exposure with 3 different screens, the standard screen, the *istD screen, and a jin finance split image screen.
the results below show the change in greyscale histogram when shooting a block wall at all F stops using the green button to set exposure.
Secondly, the reason I have a split image in the K10D is that I have a lot of MF lenses, ~10 K/KA mount and an equal number of M42 lenses. all of them are fast at their respective focal lenghts. It is a waste of time (in my opinion) to consider using fast MF lenses without a split image screen.
As for green button, the default more on the K10D and K7D (I can only assume K20D is the same) is to set shutter speed for the stopped down aperture when pressed, and DOF preview stops the lens down to the preset aperture and shows the meter reading and over / under exposure. Unless you have played somehow with the setup, it should be the same on the K20
With respect to shooting wide open, if that is all you are going to do, just leave the camera in AV mode because that is what it does with non A lenses