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Sigma 8-16mm focus issue
#1
Hi,

First post here. I recently got the Sigma 8-16mm to use with my D5100. I've had it for a couple of months and have never been happy with the sharpness. I started trying to pin it down now.

People reviewing it say it is sharp. When I use it for shots around 10 feet it is not sharp. When I use it for landscapes with tripod and release they are great, but not what I would call sharp. Shoot raw, large.



Tried both AFS and manual and LCD focus, it just doesn't seem to resolve like my other lenses (it's my only wide angle). I'm beginning to doubt myself you know, and could use some feedback. I use Lightroom to view the shots at 3:1 to try to evaluate





Thanks.
#2
Well, if you've tried to focus with liveview and this does not

change your results, you can at least exclude the AF system.

The results are (in that case) as sharp as your sample of the

lens gets.



So, you might have expectations that cannot be matched

by an ultrawide lens, or you might have a bad sample. THat is

actually very hard to tell without a sample image.



From my personal experience, most ultrawides do not (never at

no setting) get that sharp as a good telelens gets ... even if testsites

see their MTF values around the same values. The best you can do

is use a tripod, stop down, use mirror lockup and a cable release.



Just my 2 cts ... Rainer



PS: First post ... welcome at PZ !
#3
I agree, if you don't see any difference between phase AF and Liveview AF, you have no focusing problem.



You should check your lens for decentering.



Be aware that, there are sharp lenses and ... sharp lenses. If you compare the 8-16mm with, say, the Tokina 11-16mm on regular sized pictures (even full screen, HD screen only require 2 MPix), the 8-16mm will look much sharper because it is extremely contrasty.

If you look at 100% crops, the 11-16mm pictures will appear much cleaner. It depends on what you're looking for. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Wink' />



From what I saw 8-16mm is very similar in "100% crops sharpness" to the Sigma 10-20mm f/4-5.6, but with a much better contrast, better colors, much better coating.
#4
[quote name='Rainer' timestamp='1341426858' post='19318']

Well, if you've tried to focus with liveview and this does not

change your results, you can at least exclude the AF system.

The results are (in that case) as sharp as your sample of the

lens gets.

So, you might have expectations that cannot be matched

by an ultrawide lens, or you might have a bad sample. THat is

actually very hard to tell without a sample image.

From my personal experience, most ultrawides do not (never at

no setting) get that sharp as a good telelens gets ... even if testsites

see their MTF values around the same values. The best you can do

is use a tripod, stop down, use mirror lockup and a cable release.



Just my 2 cts ... Rainer



PS: First post ... welcome at PZ !

[/quote]

Ah, I appreciate your reply. I have one general question about it though...when you say it might be a bad sample, what exactly does that mean? How do I know? Does a bad sample mean it should be sent to Sigma? Is there a way to test it myself?



Thanks, this is a great forum!
#5
[quote name='Youplaboum535' timestamp='1341495203' post='19330']

I agree, if you don't see any difference between phase AF and Liveview AF, you have no focusing problem.



You should check your lens for decentering.



Be aware that, there are sharp lenses and ... sharp lenses. If you compare the 8-16mm with, say, the Tokina 11-16mm on regular sized pictures (even full screen, HD screen only require 2 MPix), the 8-16mm will look much sharper because it is extremely contrasty.

If you look at 100% crops, the 11-16mm pictures will appear much cleaner. It depends on what you're looking for. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Wink' />



From what I saw 8-16mm is very similar in "100% crops sharpness" to the Sigma 10-20mm f/4-5.6, but with a much better contrast, better colors, much better coating.

[/quote]

Right, don't think it's focus, and maybe my expectations are too high.

Thanks.
#6
Maybe you can post a few sample images?
#7
well i have the 8-16 for a 7D that seems to be sharp everywhere....with my first copy of the lens the autofocus died but the nice shop replaced it quickly, and the replacement is better than the first copy i think - i posted this shot from my first copy of this lens some time back -

[Image: i-JJ359nq-O.jpg]

here is a more recent shot that is in a rough and ready (unfinished) state from that first lens - "a dogs life" - the dog and it's human are sharp, the remainder of the shot i'm fiddling with -

[Image: i-xxr5CBh-O.jpg]
#8
[quote name='Bobcity' timestamp='1341531753' post='19331']

when you say it might be a bad sample, what exactly does that mean? How do I know? Does a bad sample mean it should be sent to Sigma? Is there a way to test it myself?[/quote]



Well, if you start with the manufactoring process, you will see, that albeit all

lenses of a type are produced from the same elements, but even those elements are

not exactly identical. They are just "very similar" and follow their specification within

reasonable limits. ... Now putting together such elements that are nearly the same,

would already lead to lenses that are only nearly the same, but within the assembly

process, there are tolerances as well. So in the end, of a batch of 1000 lenses, you

might have a few where all the tolerances add up in one direction such that the result

is outside the desired limits.



There is a variety of different defects a lens can have, some of them easy to detect like

unusual amounts of dust and dirt in a lens, uneven coating, scratches ands the like. The

most prominent one is "decentering" ... means the lenselements within a lens are not

in the positions they should be ... one or more are somehow out of their theoretically

optimal centered position ... shifted, rotated, tilted into a different position.



Klaus has, somewhere here in the forum, a link to a relatively easy decentering test.



But you will also find testdescriptions if you google for "how to test decentering".



My method ist as follows:

- Use a tripod.

- Test with focus set to infinity.

- Test against an object, that has good contrast and shows some fine detail.

- The object needs to be in a distance that allows infinity focus.

- Either set focus to infinity manually, or focus on the object, then switch to manual focus.

- Set your aperture and exposuretime manually, or at least use a fixed aperture setting.

- Take 5 shots ... one shows the testobject in the center, the other 4 show the test object in the 4 edges.

the switch to MF was done to have all 5 shots done with the same focus-setting.

- the image showing the object in the center will very likely show the highest sharpness.

- the corners will only come near this sharpness when you stop down. The important thing

is, that the corners show an equal loss of sharpness (and eventuelly contrast). If one corner

is much better or worse than the others, this is an indication for a centering defect.



With that, you should be able to see if there is something severly wrong with your lens.



Rainer
#9
[quote name='Bobcity' timestamp='1341531866' post='19332']

Right, don't think it's focus, and maybe my expectations are too high.

Thanks.

[/quote]



This lens gets stellar results in Photozone test. If Klaus or Markus reads this message, I would be very curious about the influence of the global contrast of a lens on its MTF results.

Since MTF relies on a measurement of contrast, it should be strongly affected by the contrast, which would explain why this lens, while getting high MTF scores, is not as sharp as one would expect.



However this is for a correctly centrered lens. If your lens is decentered, you should have it fixed or replaced.
#10
[quote name='Bobcity' timestamp='1341531866' post='19332']

Right, don't think it's focus, and maybe my expectations are too high.

Thanks.

[/quote]



Yet this lens gets stellar results in Photozone test. If Klaus or Markus reads this message, I would be very curious about the influence of the global contrast of a lens on its MTF results.

Since MTF relies on a measurement of contrast, it should be strongly affected by the contrast, which would explain why this lens, while getting high MTF scores, is not as sharp as one would expect.



However this is for a correctly centrered lens. If your lens is decentered, you should have it fixed or replaced.
  


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