Ian, I've got one of these, which I have had for a couple of years. I've been told by two people that there has to be a filter on all the time as it is part of the optical formula. However:
1. I got a Russian colleague to read the manual. He says it makes no mention of the filter being necessary. 2. The optical diagram in the manual doesn't show the filter as part of the lens layout. 3. I took some early pictures without the filter before I 'knew' and they look fine to me :-) I can't tell you whether it hits the mirror on the PZ-1P, but I know from experience it hits on the ProgramA/Program Plus. That probably means the same for all the M-series cameras since they have a similar chassis? It doesn't hit on the LX. However, on mine I've ground away part of the very thick aluminium filter rim on the clear filter and it now misses the Program A mirror. Optically the lens is very good; very little distortion and plenty contrasty IMHO. The downsides are the mirror problem and the fact you can't easily mount filters on the front (square 'Cokin-P' and blu-tak works!). I also had a little bit of bother with the aperture position sensing lever being thinner than the Pentax version and missing the arm in the camera. Bent it outwards a bit and it was fine. I haven't used it as much as I thought I would (I aquired a Pentax 24mm f/2.8 6 months later and then a Pentax 67 system 6 months after that). I really must do some tests with it - in fact I'll try and get out this w/e! HTH? Chris
