Originally posted by thibs No, IMO another crop would be a mistake. Either keep 1.5 or go FF.
I agree.
Also, SR should be possible with the current mount and FF.
The K mount register distance is 45.46 millimeters (same as M42), its unobstructed diameter 44.8 mm, its obstructed diameter 47.8 mm, and FF diameter 43.27 mm.
The K mount was specifically designed for FF and fast lenses up to f/1.2. I see no problem to have this mount with FF and SR. Maybe, the outer corners see slight extra vignetting during SR with some lenses but still should be fine.
It is an advantage, actually, because it makes compact FF DSLR possible/likely other vendors would have a hard time to counter.
The recent addition of the SDM contact is obstructing the mount diameter to less than 40mm. However, these are old FF lens power zoom contacts actually not creating extra obstruction for an FF sensor as they are moved slightly off the corners. It shows the headroom of the K mount.
Still, I guess that with FF/SR in mind, the SDM contacts would have been positioned different from the old power zoom contacts. They are now the mount's weakest point when it comes to FF/SR. But fortunately, there officially aren't any FF SDM lenses and maybe, there never will.
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UPDATE:
I deleted the above paragraph. I checked it out in detail and the SDM contacts are not causing an issue. I've created a drawing to verify this, cf. attachement.
The drawing is based on Bojidar Dimitrov's great Pentax K-Mount Page
Features and Operation of the Kaf2 Mount