Originally posted by normhead Most full frame sensors don't have the same pixel density as crop sensors, so it's pretty much a moot point.
IN the area of the crop, a K-3 will give you 2700 lw/ph, a K-1 will give you maybe 2100 lw/ph. The APs-c gives you more resolution in the area of the crop. If you include area outside the crop, the K-1 will give you more resolution, but probably not on your subject, using the same lens. At least of for the here and now with the pixels densities being so different.
Right. This goes back to my earlier point - that sensors with similar pixel density, regardless of format, will give very similar results for the same lens. The sensors in the K-1 and - for example - the K-5IIs have very similar pixel densities (I think?), so the same lens on both cameras should give very similar results, if the K-1 image is reproduced such that the APS-C crop portion is the same physical size as the K-5IIs image.
Frankly, I think all of this is essentially moot. Full frame lenses work great on APS-C cameras. Differences due to sensor pixel density - whilst apparent when pixel peeping - can and should be ignored. If the field of view is appropriate and the lens renders nicely, that's all the reasons we need to justify its use on crop.
A related side note: As I recall, the FA77 Limited beat (only just) the DA70 Limited when the two were tested side by side on an APS-C body, right here on these very forums