p.3 #1What do you think about this centering test?
If the above images are labeled correctly it shows how little I understand about wide angle lens distortion. I would have thought that the two bottom images would have been reversed - what is labeled as bottom left would actually be bottom right.
p.3 #2What do you think about this centering test?
Phillip Reeve wrote: RCicala wroteOne question - doesn't this method assume the field is flat? Because a) the field is almost never flat and b) most commonly the sagittal and tangential fields have different curves (most commonly one is U shaped and the other M shaped).
And I am asking it as a question, it may be that my visualization of the test is wrong. But it seems this is a test for tilt rather than decentering.I think it does and in many cases on could get sharper corners by not focusing in the center of the image but on the corners directly. But I don't think that this matters that much since im not interested in the absolute corner performance but in the performance of the corners relative to each other.
Hmm when I say a lens is decentered I mean that one side of the image is less sharp then the other. I think that is not a strict definition of decentering but the one everybody uses.
It is obvious that there is something wrong and your tests proves it. Although unlikely but to be 100% that the issue is not a misaligned camera sensor or mount I would try to replicate the problem with a similar camera with using this lens.
I had a problem with a Nikon D800E that had a misaligned sensor and thought that it was the lens a few years ago. The problem was more obvious with extreme wide angle lenses and the reason why I had not noticed it before. The camera went back to Nikon and was perfect after being serviced.
p.3 #3What do you think about this centering test?
jjohnson wrote:
If the above images are labeled correctly it shows how little I understand about wide angle lens distortion. I would have thought that the two bottom images would have been reversed - what is labeled as bottom left would actually be bottom right.
Later,
Johnny
You're not alone. I think the two bottom images' labels need to be reversed.
p.3 #4What do you think about this centering test?
sebboh wrote: Mike Tuomey wrotesebboh wrotethis method (mentioned by the OP) is the one i've always used, as it is quick, easy, and very hard to get a false positive. i can imagine manufacturers being dubious of it though due to the fact that it requires 4 separate shots and they have no way of verifying that you did not change focus between shots.
Mike Tuomey wroteEasier and less prone to false positives?
http://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-old-technique-gets-a-makeoverthis lens rentals method seems much more finicky to me since it requires a prop and misalignment could theoretically come into play.That's one advantage of the method, according to Roger C. Perfect alignment isn't required. That said, the blog article indicates it's 95% effective, and less so for superzooms and UWA. From personal use, I can say that it's allowed me to identify problem lenses twice out of about a dozen tested, which frequency surprised me.it's much easier than shooting a test chart or wall where everything is in focus, but it still takes a lot more care than the 4 corner test.
well, might be worth using the lensrental test as additional confirmation. anyway, since perfect alignment isn't required for it, the test is as simple as clamping the camera+lens on a tripod, focus on the chart and make an exposure, defocus on the chart with AF off and make an exposure, compare the two images w/r/t the center "ring," looking for the telltale assymetric defocus flare. no flare (symmetric OOF) = no problem. all indoors, no window needed, just two images.
p.3 #5What do you think about this centering test?
RCicala wrote:
One question - doesn't this method assume the field is flat? Because a) the field is almost never flat and b) most commonly the sagittal and tangential fields have different curves (most commonly one is U shaped and the other M shaped).
And I am asking it as a question, it may be that my visualization of the test is wrong. But it seems this is a test for tilt rather than decentering.
the test is just for symmetry and flatness of field is not necessary at all. it is easier to perform at large aperture with a lens that has FC if you focus on one corner (instead of the center) and shoot the other corners without refocusing. both tilt and decentering are noticeable with this test, but the test does little to differentiate between the two. tilt with no decentering will usually show more symmetry (e.g. one corner front focused and the other back focused or the top corners oof while the bottoms are in focus). results like those seen here seem to indicate decentering more than tilt because one corner is much worse than the others, but it could still be tilt with the difference between front dof and back dof causing the assymmetry between the upper right and lower left corner. all lenses have some amount of both, this test just shows how much the lens asymmetry will effect you in actual shooting.