Thanks again! Apologies, because I've expressed myself poorly in a few spots here...
Originally Posted by Richard Day 
Diffraction happens with ALL lenses, it's a fact of life!
Small apertures are required for increasing depth of field, not for increasing resolution. Nothing to do with the quality of engineering.
Yes, point taken and understood. By "engineering" I didn't
necessarily mean "Make it better", I meant "If this is as well as it can be made, only use the good parts". In other words, don't try to say "this goes to 11" when all you've done is change the numbers. Personally, I'd rather have a lens that's really good through all its range, than one that goes oh-so-high and oh-so-low BUT is still only GOOD in the middle, if you see what I mean.
Take the LL example you provided -- as Mr. Reichmann says, f32 and 45 are "unusable" -- so WHY give the lens either aperture? If the diffraction is so great that it overwhelms the DoF gains, there's no point. The
effect may be a fact of life, but the decision to allow the lens to be stopped down that far and sold as "maximum aperture f45" is not.
To be fair, since I'm only working at one distance (also true of the LL examples), it's possible I'm not seeing the whole picture. But to me, it still smacks of mindless marketing-driven specs, which may sell more lenses and cameras to the unwary public BUT does nothing to build the long-term quality and reputation of the brand.
Regarding your 50mm question. Most lens focal lengths are calculated with their focus set at infinity, not at close distances, where they will differ somewhat. Try some distance shots, there could still be some small discrepancy, but it will be much less.
Aha. Gotcha.
Back or front focus is another issue, all your lenses should be fairly close, if one is wildly out, then you should have the lens exchanged or serviced. If all your lenses are out by a similar amount, it points to faulty AF calibration of your camera, again it should be sent for calibration service.
Yes, that's what started me on this whole thing -- however, I'm happy to say that after a whole day of tweaking the Service Menu AF corrections, I was confident enough to do a four-hour shoot last night with NOTHING but the DA* 16-50, and I'm quite happy with the results. Of course, working with a diffused flash instead of straight bar/club lighting made a huge difference, I'm sure...
