By coincindence I have just been considering the same issues as "unregistered" in respect of using and metering M42 lenses on my k-r.
I had previously made a post in another thread about this
https://www.pentaxforums.com/forums/pentax-dslr-discussion/196182-custom-k20d...ml#post2254771
and PM'ed other members (more specifically relating to the use of split prism screens and their effect on metering)
The consensus of posts and responses that I read seemed to be that the high levels of exposure compensation I was needing (as much as +2 1/2eV) was simply inherent in using MF lenses, so at the time I just soldiered on.
But then I read a lens review by veteran member georgweb
(Carl Zeiss Flektogon 35mm, first review at page bottom)
Carl Zeiss 35mm F2.4 Flektogon MC Lens Reviews - Carl Zeiss Lenses - Pentax Lens Review Database
in which he refers to the "aluminium foil trick". Hang on a, minute, I thought...
I have three M42 lenses: takumar 55mm, helios 58mm, Panagor (kiron) 135mm. Pics taken with and without foil between the lens flange and the camera mount
(thus shorting the contacts) tell the story (the story being exposure: I just snapped what was convenient while sat on the sofa!) Aperture priority (A), no compensation so as the camera sees it. The RH crops with foil are much more accurate, just slight ~1/2-1eV underexposure attributable partly to the natural tendency for MF lenses to require a bit of +eV,
and partly to my use of a split prism VF, which I have roughly estimated from use with a number of lenses as tending to cause +1/2 eV needed.
The verdict is clear: using M42 lenses that don't short the contacts throws out the cameras metering a LOT.
Now what I would like to know is which contacts, and a practical solution....simply sanding the paint off the lens flange seems most practical, attempting to stick on foil well its just going to come off, ripped eg by the locking pin..
Any ideas, practical experience?