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02-26-2013, 08:05 AM   #1
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lens corner/edge consistency question

We often read lens tests that discuss corner or edge results, but almost always there is only one discussion - as if there is only one corner/edge. For those who have tested their own lenses, what's a reasonable expectation for corner/edge consistency? No noticeable difference betwen corresponding edges or corners at, say, 100% with 6MP? 10MP? 16MP? 36MP? Wide open? Two stops down? Maybe at a diffraction-limited f22/32? Even back in the old Takumar days, I recall many lens test reports stating "centering was near-perfect", which seems to imply that all the edges/corners didn't produce equal results. Of course most of us couldn't examine our pictures then the way we can now, but presumably design and manufacturing should have been able to keep pace. Obviously we can't expect, for the most part (maybe some macro exceptions) to see center-like results from corners/edges, but my question is only to what degree it's reasonable to expect the left/right top/bottom and all the coners to be similar? Is the expectation different depending on focal length? Is it different for a "consumer grade" lens vs. say, a DA*? Or should the consistency reflect the ambitiousness of the design, so that we should expect more consistency from, say, a 35/2.4DA, than from a 60-250DA*?

Paul

02-26-2013, 08:10 AM   #2
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I don't know how relevant this is to your more general question, but the only lens I have where I have noticed asymmetry in corner softness is my DA16-45. And interestingly, it's not very consistent, so I tend to believe that it's actually the well-known wobbliness of the lens barrel of the 16-45 that makes it decentered. If I only knew of a way to hold the lens so it keeps it barrel perfectly centered - LOL
02-26-2013, 08:29 AM   #3
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What a coincidence, I just bought a lens recently and one of its edges is worse than others. I consistently get noticeable fringing (blue yellow, and red cyan) in the bottom right corner, but not in the other corners. I even took the same frame with the camera turned upside down, and the result was consistent (as in, the same edge of the lens is worse). And now I'm in a dilemma, whether I should ask for a new lens or what. Its the Samyang 14mm btw.
Thing is, most people don't notice this fringing, but I do. And I'm not sure the next lens will be better. I think this has more to do with sample variation than lens design

Last edited by Na Horuk; 02-26-2013 at 08:43 AM.
02-26-2013, 08:29 AM   #4
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There is absolutely some degree of variance between corners/edges of the frame when testing lens sharpness ... and there is sample variation even among the same lens. If you want to see a good visual representation of this then check out some of the lenses listed over at SLRGear.com ... the ones that are labeled "tested" that is. For example, they tested the FA Limited 43mm f/1.9 and found that two of the corners were softer than others:

Pentax Lens: Primes - Pentax 43mm f/1.9 Limited SMC P-FA (Tested) - SLRgear.com!

Compare that to their test of the 40mm Limited and you'll see the 40mm lens they tested had better edge-to-edge sharpness (even when both lenses are set to f/2.8).

Pentax Lens: Primes - Pentax 40mm f/2.8 Limited SMC P-DA (Tested) - SLRgear.com!

However, obsessing over tests like these is similar to pixel peeping for signal/noise ratios with image sensors. The only time you should care is if your lens is so messed up that it "obviously" impacts the image quality.

For example, I used to own the DA70 which every test I've seen shows has better edge-to-edge sharpness than the FA77 but I came back to the FA77 and got rid of the DA70 because I prefer the softer bokeh of the 77 and I wanted the ability to open up to f/1.8 if I need to.

I created the image below with a zoom lens that multiple review sites say is noticeably less sharp in the corners than in the center of the frame. However, when I show this image to people they usually just comment on how sharp and detailed the image is and no one has ever said, "Oh my God! Those corners are so soft!"



02-26-2013, 08:50 AM   #5
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Especially with zoom lenses, there is a tendency, when centre is sharp, for one edge and its corners to be more blurred if the rear element and the film/sensor plane are not exactly parallel.

A while back I repaired an old off brand zoom, and then corrected it by grinding the mount base flat by trial and error then re-shimming to register for sharp infinity.
I posted some photos and test shots at the time.
02-26-2013, 08:53 AM   #6
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QuoteOriginally posted by tibbitts Quote
what's a reasonable expectation for corner/edge consistency?
I think that it depends on the kind of photography you do.

If you're into a more pictorial sort of photography,
you may be looking for center crispness, and not be concerned about edge performance.
For this kind of photography, lenses like the Sigma 30/1.4 and Pentax FA* 24/2 are well appreciated.

On the other hand, if your photography includes genres like architecture,
or certain kinds of landscape or technical subjects,
you'll be looking for edge-to-edge sharpness,
and may not accept very much of the inevitable decentering,
or may want to avoid the more "pictorial" lenses.
02-26-2013, 09:02 AM   #7
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QuoteOriginally posted by tibbitts Quote
back in the old Takumar days, I recall many lens test reports stating "centering was near-perfect", which seems to imply that all the edges/corners didn't produce equal results. Of course most of us couldn't examine our pictures then the way we can now,
You could always examine a slide or negative under a strong magnifier,
right down to the grain.
Pixel-peeping the old-fashioned way!

QuoteOriginally posted by tibbitts Quote
but presumably design and manufacturing should have been able to keep pace.
If anything, I get the impression that modern lenses are more susceptible to decentering.

For one thing, newer lens designs often now have more elements, which may make them trickier to assemble.
And lenses have become more of a mass market item, rather than a costly luxury,
so most companies cannot afford to assemble lenses with extreme care,
or reject a significant proportion of their output.

Sometimes manufacturers even build in decentering deliberately,
in the name of "optical image stabilization."
For example, the older version of the Tamron 17-50/2.8
is often consiidered to be sharper than the newer, stabilized version.
02-26-2013, 09:27 AM   #8
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All the decentred lenses I have are zooms - a clearly faulty Sigma 17-70 (first version) that Sigma fixed without fuss; a very slightly off DA12-24 that was still quite acceptable; a grossly decentred DA55-300 that Pentax claimed was in spec but is totally unusable; and a slightly decentred DAL 55-300 that is just about usable. My two 18-55s are fine, as is my wonderful FA24-90. My primes all seem OK. So it's a bit of a lottery.

02-26-2013, 10:24 AM   #9
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QuoteOriginally posted by Na Horuk Quote
What a coincidence, I just bought a lens recently and one of its edges is worse than others. I consistently get noticeable fringing (blue yellow, and red cyan) in the bottom right corner, but not in the other corners. I even took the same frame with the camera turned upside down, and the result was consistent (as in, the same edge of the lens is worse). And now I'm in a dilemma, whether I should ask for a new lens or what. Its the Samyang 14mm btw.
Thing is, most people don't notice this fringing, but I do. And I'm not sure the next lens will be better. I think this has more to do with sample variation than lens design
Is this on a FF camera or digital. Since the samyang 14 is a full frame lens, you should see nothing at all in an APS-C camera frame. I use my 14mm on a PZ1 and have no issues. Mine is branded vivitar, but it is the same lens
02-26-2013, 06:05 PM   #10
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QuoteOriginally posted by gazonk Quote
I don't know how relevant this is to your more general question, but the only lens I have where I have noticed asymmetry in corner softness is my DA16-45. And interestingly, it's not very consistent, so I tend to believe that it's actually the well-known wobbliness of the lens barrel of the 16-45 that makes it decentered. If I only knew of a way to hold the lens so it keeps it barrel perfectly centered - LOL
Actually I've tested my 16-45, and even though the front element wobbles severely, pushing it from site to side or up/down while taking a picture seems to have no effect on sharpness, which seems surprising. But I can't produce softness by shifting that front element.

Paul
02-26-2013, 06:08 PM   #11
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QuoteOriginally posted by timo Quote
All the decentred lenses I have are zooms - a clearly faulty Sigma 17-70 (first version) that Sigma fixed without fuss; a very slightly off DA12-24 that was still quite acceptable; a grossly decentred DA55-300 that Pentax claimed was in spec but is totally unusable; and a slightly decentred DAL 55-300 that is just about usable. My two 18-55s are fine, as is my wonderful FA24-90. My primes all seem OK. So it's a bit of a lottery.
What was your experience trying to have Pentax repair the lens? It's discouraging that Pentax wouldn't repair or replace the lens given evidence of poor performance, although I had somewhat the same experience with Sigma. It came down to a difference of what was acceptable.

Paul
02-26-2013, 09:10 PM   #12
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QuoteOriginally posted by tibbitts Quote
What was your experience trying to have Pentax repair the lens? It's discouraging that Pentax wouldn't repair or replace the lens given evidence of poor performance, although I had somewhat the same experience with Sigma. It came down to a difference of what was acceptable.

Paul
Paul, Pentax had already 'repaired' the lens. It was after the repair that I sent it in for a second time (to Pentax in Tokyo via our local agent) and complained. They just sent it back, having checked it (allegedly), saying it was within spec. A complete joke. Mind you, that was when they were going through their greatest period of corporate upheaval. I wonder if that was a factor. Perhaps the guy was about to be retrenched ...

Yes, of course you are right as to what is 'acceptable'. It's very subjective, which is why I mutteringly accept a bit of decentering with the other lenses I mentioned earlier. It's not enough of an issue considering they are not high-end lenses. But in this case I don't think anyone could reasonably claim that the performance is reasonable, even stopped down. Anyone want a paperweight? (Although it's a bit big for that.)

Tim
02-27-2013, 02:01 AM   #13
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QuoteOriginally posted by tibbitts Quote
Actually I've tested my 16-45, and even though the front element wobbles severely, pushing it from site to side or up/down while taking a picture seems to have no effect on sharpness, which seems surprising. But I can't produce softness by shifting that front element.

Paul
Interesting. I'm beginning to think that my lens is slightly decentered after all, since the softness always seems to appear on the right-hand side @ 16mm. I wouldn't be surprised if it recently got slightly decentered - it got hit when I fell some months ago and slammed my camera bag so hard in the ground that the protective filter was crushed. It's a pity, but for the widest end I hope to replace it with a DA15 anyway.
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