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12-16-2008, 07:29 PM   #1
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HELP! Is this normal?

OK so i got my Sigma 17-70mm and i must say it's a pretty awesome lens, nice and sharp and my pictures certainly look more vibrant and clear compared to the kit lens.

My only issue is this:

http://www.flickr.com/photos/pentaxftw12/3114966648/sizes/l/

Check out the details at the far left and right of the image...WTF is that :|
Is it called vignetting? or chromatic fringing? I don't know...it's some sort of yellow/blue light along the edges of sharp lines close to the sides of the image. The picture was taken at 70mm and almost at it's closest focusing distance.

Is this normal? If not...should i send it into Sigma for recalibrating? If i do decide to send it in how to i describe this problem, what is it called?

[PS view 'Original' size for a closer look]


Last edited by PNTXFTW12; 12-16-2008 at 08:06 PM.
12-16-2008, 07:49 PM   #2
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Looks pretty blurry on the edges, if you were hitting minimum distance, it's possible the field curvature could do that.

Klaus from photozone.de explains (Nikon's 24-70 AFS lens)

"Field Curvature
(updated: 03/15/2008) Field curvature refers to the shape of the focus field. In a perfect world we would assume that the depth-of-field (the sharpness zone dependent on the object magnification and focus distance) starts at some focus distance x in front of an focused object and ends at some distance y behind. The difference between x and y is called the depth-of-field. In the real life there's no perfection though and the focus field can be rather distorted (as opposed to flat) with consequences for the depth-of-field. Normally this isn't overly field relevant but it is for the Nikkor 24-70mm f/2.8. At 24mm the focus field is curved (thus called field curvature) - at the image borders the focus field turns inwards (towards you). In infinity focus situation this may cause some focus blur for distant object located at the edges of the image field. It is also relevant for flat objects. See also THIS EXAMPLE - you may notice that the center is perfectly sharp here whereas the borders are soft."

From your sample it looks like it might be slightly off-centre. The colour cast on the left, I don't know.
12-16-2008, 07:52 PM   #3
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Hmm that makes sense and all...but it's pretty noticeable on mine.
Theres a group on Flickr dedicated to that lens and ive looked through some macro shots and i haven't seen any that have it as bad as mine.
12-16-2008, 08:00 PM   #4
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Unless I'm mistaken, field curvature should show itself evenly, therefore left and right should at least be affected the same. I have never owned a decentered lens, but by definition, that looks like decentering to me.

By the way, good choice of image to show this. Other than a standard grid, that pattern shows the blur quite well.

12-16-2008, 08:05 PM   #5
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thanks frogroast lol
but what's the verdict guys? Should i send it in or not
im so confused...i'm still kinda new to this stuff...you guys have to help me out here :P
12-16-2008, 08:13 PM   #6
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Take a few more shots, and see if it improves at a smaller aperture (try f/8 and f/16)

If you got it from a store, try another sample, if online, email them the photos and ask for an exchange.

Unfortunately 3rd party makers aren't famous for quality control, even when they produce some great lenses. But Pentax had similar issues with their 16-50 premium zoom.
12-16-2008, 08:13 PM   #7
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IMO, it should not look like that, send it in, it seems very decentered to me.
12-16-2008, 08:28 PM   #8
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Definitely send it back. It would be better if you could walk up to the store and try another lens on the spot, but if you got it by mail, you will either have to take a chance at another lens or just return it for a refund.

If you decide to get another and they insist you return it first, take a picture of the lens serial number. Don't want to end up getting the same one back, lol.

12-16-2008, 09:13 PM   #9
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alrighty! I'll head over to Henry's and see what they can do about it first!

[PS henry's=where i bought it]
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