Thank you!
Also, I should have said "longitudinal chromatic aberration," not lateral. And I thought about it... yes, the lens is probably NOT under-corrected for spherical aberration, per se, may be it's just almost ALL 'fast fifties' suffer from their spherical aberrations wide-open or near wide-open. I doubt correcting for incidental spherical aberration issues in a fast lens, wide-open, is a realistic optical engineering goal. That would be something you pay thousands of dollars for. (How do the Zeiss Otuses perform in this regard?) Clearly this lens appears to perform to the highest marks when stopped down.
Loss of contrast, Long. CA, these together with focus shift are quoted as the hallmarks of spherical aberration by H. H. Nasse in his "How to Read MTF Curves" white papers written for Zeiss.
(I don't want to hot-link these papers. Search them up. They're fantastic reads for people who did okay in math and science in high school or had some college, and generally enlightening in their observations and conclusions, otherwise. Also, his "Depth of Field and Bokeh" seems like it would be an absolute smash hit among the various photography forums.) So I presumed all the reports of focus issues had something to do with this.
Most of the reports of autofocus problems with the DA* 55/1.4 seem to be with back focus and users dialing in corrections for that. That could indicate focus shift, which, according to the articles on diglloyd, almost always manifests as back focus. And he tested all the DA Limited primes (except the DA* 55) and found them all to exhibit focus shift. Which is kinda wild considering how slow they each are wide open. A compromise for the physical form factor of the lenses? Funky interactions with field curvature? Or maybe he's just got a severely critical eye, he is a militant wizard about sharpness. Then you have the highly visible reports of people trying a dozen DA* 55's and seeing all copies focus totally randomly, or having apparently tilted elements.
I think the biggest thing that triggers my curiosity, is the DA* 55/1.4 being marketed as specifically a "portrait lens" and being compared to the FA* 85/1.4. The FA* is reported to consistently front-focus and to "behave poorly" focused at infinity, though at what distances and what exactly that means, I don't know. And certainly it's tuned to be bokeh-licious, which would have something to do with engineered under-correction of spherical aberration, if you consider the writings of H.H. Nasse.
I also have a DA 20-40 Ltd., and I don't wonder about that lens, because it isn't marketed as being application-specific. It's just a solid all-around performer with no glaring weaknesses, a great "adjustable normal" walkaround lens, bomb-proof in handling flare, seemingly no loss in contrast working into the light, sharp as you need it to be stopped down, not horrendous wide open, just a little mushy on the edges like any other lens. I carry it in-hand on the street where it's easy to handle and I can bang it off lamp posts and not worry too much, spill some milk on it etc.
Right! Not "in-spec" per se, we know how that goes with copy variation (thanks in no small part to the lensrentals blog) and factory service centers, I mean just performing in-character. Whaddaya mean, we don't yet have a forum group that's gone in together on the $420 Imatest "Lite" version (with printed test chart!) and gone all sortsa crazy testing Pentax lenses? That might have to change! (Though you have to pay megabucks for the full version that can test MTF other than MTF50. MTF10 and MTF90 at 10, 20, 40 lp/mm would make great fodder for arguments.)
Can someone enlighten me as to what is meant when it's said a lens "doesn't perform well at infinity?" Are we talking a nose-dive in the MTF, some visually apparent optical aberrations, crappy autofocus at the near-infinity mark, etc? It's not something you hear about much from people who spend actually-reasonable amounts of money on glass I guess.
I did some hand-held aperture series with a brick wall and an actual scene. I'll bust out the tripod to tighten things up, and set up my metal ruler under good light indoors and posts some test results myself. So far I at least don't appear to have any centering issues. I focused manually stopped-down in live view, I'll have to work a few variations of focus technique, too. Hit me up with those anecdotes and I'll come back with the patent search tomfoolery I've done.
