Originally posted by Wild Mark I do not know for sure, but, I am 92.93% sure that the mirror will not hit the rear element. The M42 lens is designed to fit natively on pentax M42 FF cameras and with a simple adapter fit on K mount (with no change in registration distance). The problem emerges in cameras with a shorter registration distance such as canon. This is why canon users 'hack' the rear assembly of K mount lenses and use specific adapters to avoid damaging the rear element.
But I am 'unsure' only because I have not tried the combo you speak of. I rely on the idea that M42 lenses are meant to work as designed on K mount cameras with the right adapter.
This was my original thought, also .... but then I thought, "Why trust years of Pentax engineering when I can actually look for myself?", so I took the lens off my K-30 {currently my only working K-mount camera}, turned it on, and put it into LV mode {which has the effect of lifting the mirror}. When fully lifted, the mirror comes out to the ridge I marked with an
orange arrow; it occurs to me that those clever Pentax engineers designed the camera so that ridge would prevent a lens from protruding into where the mirror could hit it, but then I took some measurements; the ridge is approximately 5mm from the mount edge, and my Super Tak 50mm lens protrudes 4mm behind the mount when focused at infinity, so I guess this is safe enough.
{and I'm guessing that all recent consumer-level Pentax cameras - including the K-S1 - use the same mirror-box design; engineers tend to be conservative, which means, amongst other things, not redesigning something which meets current needs}