De-centering Defects of Pentax Lenses

Klaus Schroiff of the PhotoZone recently has cancelled his tests on both of the two latest DA* zooms, owing to the pronounced "centering defects" found. In fact, he has already found so many and too many similar defects with Pentax lenses during his tests on Pentax lenses carried out up till now. With those centering defects found, Klaus either re-acquired another sample from the PhotoZone community or he simply cancelled the tests, which is indeed a rather disappointing thing, especially when this keeps repeatedly happened, very unfortunately.

A search of the term "centering defect" in the PhotoZone forum datebase using its search function will reveal how many times this problem has been appeared and reported during the course of his testings in recent months. I'm actually not being totally surprised for having alignment issues with Pentax products from my own "unfortunate" experience, especially for the recent Pentax products which were made and are being made outside Japan.

Nevertheless, I am a bit sceptical about if it is really about the poor QC of the Pentax lenses instead of the mis-alignment of Klaus's K10D body instead. Coincidently, I have asked Klaus the similar questions more than twice in different posts in which he reported the problem when he tested different Pentax lenses (which I can see many of the conversations again when I have done the search on the keywords "centering defects". Below were some of his answers to my questions:-

http://www.photozone.de/active/forum/ShowMessage?ID=108899021463946422146394641A

"This is very unlikely. There were no significant centering defects in the Tamron 70-300, Cosina 100 and the Sigma 10-20 tests - these were among the most recent test runs in Pentax land."

http://www.photozone.de/active/forum/ShowMessage?ID=108552321463980702146398065A

"An asymmetrical centering defect isn't all that severe from a testing perspective.
If I reject a lens it is mostly because the _center_ portion shows both sharp (although suboptimal) edges as well as "edge shadows". In the real life this kind of defect shows up as weak contrast but not as asymmetry."

Here is a capture of the *image center* for a chessboard like resolution chart pattern:-

http://www.photozone.de/active/forum/ShowMessage?ID=10829312146400715A

As we can see from the above diagram, asymmetricity or the phenomenon of directional "edge shadows" does really exist. As Klaus has mentioned in so many of his Pentax lens review and posts about his tests that there are centering defects in common (more or less), objective facts tell me that I should admit that there is a problem with even the Pentax optics, despite I have already tried to think objectively about any other possibly cause and asked for times if "it should be the body". (Well, I do not have very good faith or impression on the quality control and perfect alignment of the K10D body but I can't imagine even a bundle of different Pentax lens models could have serious alignment problem - even for the optical center).

Very interestingly, MTF tests like those conducted by Photodo indeed can already show the presence of centering defects, as cleverly pointed out by this PZ poster:-

http://www.photozone.de/active/forum/ShowMessage?ID=1082931214640066121464006602146400658A

Now, the DA* 16-50 and DA* 50-135 samples which have been got from the street have very significant and pronounced centering defects. Despite the tests have been cancelled and Klaus will not publish the test results formally anymore, he is yet kind enough to post briefly the unpublished results in his forum:-

http://www.photozone.de/active/forum/ShowMessage?ID=10889902146394636A

Well, as we can see, actually both DA* perform not so well especially for the poor/low resolution at wider apertures and at the image borders as well as the huge amount of CA present. Whilst the centering defect, which is found to be existent in both DA* lenses, could affect image quality and "spoiled the MTF figures a bit", which has been verified, here, possibly the lenses' optical performances are not that good by themselves. As remarked by the tester, the inferior CA performance is just "Tokina alike".

Last but not least, as those lens samples obtained are new products which have passed the factory QC and those are acquired from the streets, we may think it in the following two additional possible ways:

1. The de-centering amount is just within Pentax's factory specs but should not be considered as a defect by Pentax's own standard - but then Klaus compared the Pentax lenses from Canon, Nikon etc. or even to Sigma and Cosina (on the same K10D) but he found that particular Pentax lenses are worse, including even the latest "luxury" DA Star zooms;

2. Those samples put under test are just lemons and they leaked through the Quality Control system/net from the factory - if so, I'm afraid that the QC system of Pentax is really rather ineffective or even useless as there are so many bad samples out there as picked up in the streets and ultimately been borrowed/bought by Klaus and very unfortunately even been put to the lab test.

All in all, my opinion is simple: for lower priced lenses, such defects are more acceptable as the customers should get what they paid (for). However, for "Star" lenses that marked with a new golden "Star" label on them, with a "Star" price, shouldn't we expect a "Star" quality also, in terms of *real* stringent QC tolerance and inside performance as well.. But, so far from what we can see from all those specific adverse user or even test reports of the new DA Star lenses (which are shown together with quite some evidences), I'm really afraid that our above assumption has not been proved valid and maybe even just wishful thinking of we Pentaxians. Really sad.. :-(