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08-25-2010, 10:27 AM   #1
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Problem on K7 with sigma lenses

I have a K7 with WR zoom lenses and several other lenses inherited from my previous 35mm equipment, I'm fully satisfied except for one of them; the Sigma 28-70 f/2.8 EX, which has a problem: the image shows a sort of "halo"/"glare" expecially at full aperture (f2.8), it decreases when closing diaphragm and disappears definitively at f/8. Please look at the attached image. The first series shows the same photo on the sigma at different aperture, the second shows same photo done with 3 different lenses all at full aperture, two of them are inherited from 35mm, the last is the WR zoom.

My first thoughs was oriented on a focusing problem, so I have played a little bit with focus adjustment but it does not seems to produce any changes in the problem.
I really don't know what could be...

anyone of you can help ?

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08-25-2010, 01:21 PM   #2
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You may have simply been off on the focus a little bit- were you using AF or MF? If you were indeed using AF, and focused on the chart, then either your lens is suffering from front focus or back focus, or it's somehow been damaged.

Keep in mind that image quality is typically significantly worse when the aperture is opened all the way, but it shouldn't be bad enough to the point where it's unusable.

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08-28-2010, 01:26 AM   #3
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Hi Adam

First of all thx for your reply.
As for you inputs I've done some other test on focusing and I'm convinced that it is not a focusing problem. I've attached another image with shots on a focusing chart at different AF regulations (-10, -5, 0, +5, +10) and seems to me that AF works correctly and there is non front or back focusing problems.
I've also attached another comparison of a shot taken with the sigma at f2.8 and another taken with an old fixed focal pentax lens at f2 (both 50mm). In normal conditions I would expect the sigma to perform at least as the pentax... while the shots shows an unacceptable behavior of the sigma... the image talks by itself.

I think that sigma lenses may be difective... don't know what kind of problem it may be but doesn't seems to me a focusing problem... do you still think it may be a focusing problem ?
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08-28-2010, 11:15 AM   #4
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I don't know about "defective" - most lenses (especially fast lenses) are a bit soft wide open, and what you are showing here really doesn't seem out of line with how a lot of other lenses perform wide open. But I don't know that lenses in particular - maybe it's supposed to be better than average wide open. Maybe do a search to see if you can find someone who has posted crops from their copy at different apertures so you can compare?

BTW, hopefully you are using a hood, and no filter?

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