See my Pentax K1 wish list at B&H Photo.
See Pentax K1 Coverage.
Sanjin D writes:
Earlier lenses that mostly covered an APS-C crop sensor don't deliver such a nice performance on a full frame?
DIGLLOYD: In a nutshell: it all depends.
I’m told by one reader that certain Pentax APS-C lenses may in fact be full frame designs—not sure if this is accurate but it could be.
For a good sample of what a lens designed for APS-C might do, see the three “Usage...” pages for the Zeiss Touit 12mm f/2.8, Zeiss Touit 32mm f/1.8 and Zeiss Touit 50mm f/2.8 in my Guide to Mirrorless. I evaluate usage of the Zeiss Touit lenses on the full-frame Sony A7R in a variety of aspect ratios up to 3:2 full frame.
Most APS-C lenses can be used to substantially extend the imaging capture area, since the lens “covers” (projects an image circle) which is quite a bit larger than the APS-C area. The APS-C Pentax HD Limited lenses are good candidates as are many others.
However, even if a lens designed for APS-C covers the frame, its quality may drop off outside the APS-C area, or at the least, or behaviors like field curvature are likely to grow, as well as various aberrations. Bottom line is to try it, and to expect to have to stop down to f/8 or so for best results, since the outer areas are not likely to perform well at wider apertures. Also, stopping down generally grows the image circle just a bit (in terms of usable quality).
Finally, focus distance has an impact: the image circle is smallest at infinity focus and typically enlarges at closer range (unless the lens plays the trick of shortening the focal length).