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Methods for testing lens decentering?
#1
A while ago Klaus posted a method for testing lens decentering. The problem is that I can't locate the thread. Anyone happen to have it in their bookmarks?



My own crude method is to shoot a far away cityscape from a high viewpoint, so that everything corner-to-corner is practically at infinity. If sharpness distribution is asymmetrical then I consider the lens to be decentered. The method Klaus posted relied on some test target that would potentially allow it to be performed at the store, right after (or during) the purchase of the lens. It's also more practical when meeting a seller of a second hand lens.
#2
[url="http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check"][url="http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check"][/url][/url]



You mean this one I guess:



[url="http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check"]http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check[/url]





[url="http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check"][url="http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check"]

[/url][/url]




#3
Lens decentering generally is much more obviosu at close ranges than at long ones.



My method of testing is to shoot at different apertures, at different zoom lengths if it is a zoom lens, of a more or less evenly lit, flat object preferably with plenty of detail, like a brick wall or newspaper flat on the ground or so, head on, camera sensor parallel to the object.

I don't worry too much about sharpness when doing this test, but whther the corners and edges are equally sharp. With decentering you will often find that one edge, corner or a part thereof is clearly less sharp than the rest. And you may also find that it is possible to get that one area sharp by manually focusing, but then the rest of the image will be unsharp.



Personally I have encountered this only twice so far, once with an 85 F/1.8 and a TS-E 45 <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



BTW, I have seen a lens where it was a weird streak from the mid quarter left to the centre of the image once; that was a zoom lens. I guess with zoom lenses decentering can well be more complicated in nature due to the many different elements and their complex movements than with primes.



Generally, you will likely find that the area affected by decentering is anywhere from about 2 to 20% of the image, and in bad cases it will not disappear when stopping down. With the 85 I mentioned above it was still visible on APS-C at F/11, with the TS-E it disappeared at F/8.



In short, check for an even fall-off in sharpness (and vignetting, also an indicator) from the centre towards the edges and corners. Also check at various apertures, to see how it behaves wide open and stopped down.



Once you have seen decentering once or twice, you'll generally recognize it with just a few shots.



HTH, kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#4
[quote name='PuxaVida' timestamp='1320227920' post='12620']http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check[/quote]



Thank you, that's the thread I was looking for.
#5
[quote name='wim' timestamp='1320229229' post='12621']

Lens decentering generally is much more obviosu at close ranges than at long ones.[/quote]



The problem is that at close range what seems like decetering may actually be insufficient parallelism (between the camera and the flat subject). Shooting a subject at infinity solves this nicely, and from my experience it does allow spotting decentering.



Quote:My method of testing is to shoot at different apertures, at different zoom lengths if it is a zoom lens, of a more or less evenly lit, flat object preferably with plenty of detail, like a brick wall or newspaper flat on the ground or so, head on, camera sensor parallel to the object.



I can see the point of testing a lens using various aperture values, but decentering is mosly evident when shooting wide-open, so for this specific test I don't really see the benefit in shooting stopped down.



Quote:I don't worry too much about sharpness when doing this test, but whther the corners and edges are equally sharp. With decentering you will often find that one edge, corner or a part thereof is clearly less sharp than the rest. And you may also find that it is possible to get that one area sharp by manually focusing, but then the rest of the image will be unsharp.



Yes, this is my experience too.



Quote:Generally, you will likely find that the area affected by decentering is anywhere from about 2 to 20% of the image, and in bad cases it will not disappear when stopping down. With the 85 I mentioned above it was still visible on APS-C at F/11, with the TS-E it disappeared at F/8.



This is quite different from my experience. In the cases where I've seen obvious decentering the effect reduced significantly stopping down by a couple of stops. You can also see it in the slrgear.com lens tests (some of the lenses have serious decentering wide-open, but become pretty even stopped down). I wonder what's the experience of other testers.
#6
[quote name='boren' timestamp='1320394917' post='12661']

The problem is that at close range what seems like decetering may actually be insufficient parallelism (between the camera and the flat subject). Shooting a subject at infinity solves this nicely, and from my experience it does allow spotting decentering.[/quote]

There is a very simple solution to this, namely using a tripod, and pointing the camera down at a newspaper, using a double or triple spirit level in the hot shoe to make sure the camera is aligned properly. Provided the floor is level, or a table if you prefer to use that, this works very well and is easy to set up even in cramped spaces. Actually, a spirit level is absolutely essential when testing for decentering.



Furthermore, if the camera isn' t aligned properly, the out of focus effect is very regular, unlike decentering. With alignment out of order, an edge or corner will be OOF, with decentering IME it rarely is completely regular. It often is part of an edge, or part edge, part corner, and often it is not a regular slowly dissipating patch, especially not with zoom lenses.



BTW. considering DoF, it may not be easy at all to spot decentering at infinity. YMMV, of course.



Quote:I can see the point of testing a lens using various aperture values, but decentering is mosly evident when shooting wide-open, so for this specific test I don't really see the benefit in shooting stopped down.

It is to see if it really is decentering or not. Essentially the pattern shouldn't change, just get smaller. I think it is worthwhile to do, also in order to determine how bad it really is. Again,YMMV.

Quote:Yes, this is my experience too.



This is quite different from my experience. In the cases where I've seen obvious decentering the effect reduced significantly stopping down by a couple of stops. You can also see it in the slrgear.com lens tests (some of the lenses have serious decentering wide-open, but become pretty even stopped down). I wonder what's the experience of other testers.

I didn't say it stays the same - I only mentioned it was still visible at F/8 or F/11. In some cases it won't be. A significant reduction is relative anyway. What one finds significant, another person may find small. Personally, I abhor the exteme corners of a 17-40L below 20 mm on FF, amongst others because I compose for the entire frame. Others may be very happy with it at the short FLs. In short, YMMV.



The 85 F/1.8 I was talking about, showed about the same decentering softness at F/1.8 as it did at F/8, and never really provided an image which was totally sharp over the whole frame of a flat object, whatever the aperture. It was either the decentered area that was sharp, or the rest, but never all. So yes, this was a really bad case. Also, I was allowed to take the lens home for two weeks, so I could test it at leisure, on different subjects, using different methods - the results stayed the same. Same with the TS-E. That had less problems, but I noticed it when doing a round of tets shots when shooting buildings, in non-shifted, non-tilted mode. And her eI tested with small amounts of shift and tilt as well, just to see if it wasn't the mechanism slightly out of alignment.



Other than that, I have come across a few other lenses with decentering, but none as bad, relatively speaking, as these two examples.



Regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#7
I used the method put forth in the Lens Test FAQ section [url="http://www.opticallimits.com/lens-test-faq"]Here[/url]. I followed the directions exactly and was able to determine that my lens did in fact have a centering problem. I suspected it had one, but this test gave me the confidence I wanted before sending it in for repair. The repair facility confirmed my findings and repaired the lens.



If I forgot to do so earlier, thanks to whomever included the test instructions.
#8
[quote name='wim' timestamp='1320229229' post='12621']

Lens decentering generally is much more obviosu at close ranges than at long ones.[/quote]



I assume that shooting at long distance is used to test the tilting problem of a lens. If a lens is perfectly aligned itself but the optical axis is not perpendicular to the sensor plane (I don't know what is the factory tolerance for this) then a tilting effect will be easily seen at long distance.



Frank
#9
[quote name='Bryan Conner' timestamp='1320474741' post='12694']

I used the method put forth in the Lens Test FAQ section [url="http://www.opticallimits.com/lens-test-faq"]Here[/url]. I followed the directions exactly and was able to determine that my lens did in fact have a centering problem. I suspected it had one, but this test gave me the confidence I wanted before sending it in for repair. The repair facility confirmed my findings and repaired the lens.



If I forgot to do so earlier, thanks to whomever included the test instructions.

[/quote]



An alternative approch is to use the Zeiss Siemens Star Chart as described in http://www.lensrentals.com/blog/2012/05/...a-makeover



Frank
  


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