Originally posted by BigMackCam Perhaps I'm incorrect, but if a lens covers the full image circle with reasonable optical performance and only soft vignetting (as opposed to the hard cut-off we get from, say, a circular fish-eye) then it can be considered compatible with the format.
No, you're not incorrect. But there are other issues involved. Performance is not only subjective for users, but for the manufacturer as well. Pentax has their own ideas of what qualifies as a DFA lens, and they may be particularly fastidious as to what qualifies either at DFA star lens or a DFA limited. The DA* 55 may be a legitimate DFA lens in that it covers the FF circle and covers it reasonably well—but Pentax may not regarded it's performance outside the APS-C circle as good enough to qualify for the star designation, which means they can't honestly market it as the DFA* 55/1.4.
Now I'll admit that I have very little interest in using DA lenses on FF. I have used the DA 1.4 converter on FF, but that's for critter images which don't require sharp edges and often need to be cropped in any case. I've also used the DA 35 Ltd on FF as a macro lens. It works fine at close distances but is unusable, for my purposes, at longer distances. I mostly use FF to get the highest quality landscape images
edge to edge, and I'm not inclined to compromise on that. In many respects I prefer the APS-C format for the smaller size of the cameras and lenses, so if I'm going to bother with the larger, heavier FF format, it has to pay better dividends across the entire FF frame, not just in the APS-C part of it. When I first acquired the K-1, I tried the DA 10-17 on it, and while that lens "covers" the FF frame from 14mm to 17mm, I found the performance outside the APS-C image circle to be appalling. I could capture more detail toward the edges using that lens at comparable focal lengths on APS-C. So I ended up tracking down a copy of the F 17-28 for use on the K-1, and that lens really does deliver FF performance across it's entire zoom range on the K-1 with little in the way of unpalatable compromise.