Focal length is
not the same as the Field of View of the final photo. Focal length is a lens property and is independent of camera.
DA 35mm on K-1 (full frame) will look wider than on K-S1 (APSC) because literally the edges will be cropped, cut, off the frame on the APSC camera. It simply has a smaller sensor, so the edges are cut off. This makes the field of view look more narrow. But all the other lens characteristics like minimum focus distance, bokeh character, sharpness, color rendering will still depend on the particular lens design, so there will be differences. Just the FoV will be kind of similar.
Same thing for DFA 50mm. Its a full frame lens, but it will look wider on K-1 (FF) than on APSC cameras.
DA 35mm on K-S1 will have a Field of view similar to, for example, DA 50mm on the K-1.
50mm on APSC is slightly telephoto.
50mm on FF is normal
50mm on 645 (medium format) is slightly wide angle
The only catch is that a lens for a small format might not cover the area of a bigger sensor. So some DA lenses will have black edges on FF. All Pentax lenses are FF (like F, FA, DFA series), except for some DA lenses. There is a thread "DA lenses on full frame" and some other threads as well that discuss which DA lenses work 100% on FF. All FF lenses work normally on crop sensors. You can put Medium format lenses on FF or APSC lenses, if you have an adapter, but they are really big, with slower aperture, and there will be a lot of loose light in the mirror box. This is why its generally not a good idea to mount lenses that are way too big for your format. FF and APSC are fairly close, so it doesn't matter much
Originally posted by bgscott Because DA lenses are digitally optimized for APS-C sensors, it is my understanding that crop factor would NOT come into play when using a DA series lens (or DA-L) on a Pentax cropped sensor camera.
So, the crop factor does not depend on lens. It depends on camera.
Honestly, here is the deal. If you only shoot with one format, then don't even think about the whole crop thing. It just confuses people. The only reason crop factor is a big deal is because when photographers moved from the (Very popular) 35mm film to the first digital cameras, the digital ones were all cropped. So they needed some rule to figure out the different FoV they would get on the different camera.