11-04-2011, 12:02 PM
[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.
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.
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
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 ....