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08-21-2008, 07:52 AM   #1
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Help! Is this optic normal?

Hello... I am going to buy a Sigma 17-70 and the seller posted me three pictures he made with.. But in one of them I notice chromatic aberrations.. Is it normal or this is a bad lens?
You can see the original picture shot at 17mm (compressed to 1024 x 685) and the crop from the original of the bottom left corner...
Can I buy this optic?
Many thanks...

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08-21-2008, 08:17 AM   #2
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my personal opinion (2c) is that w/ Sigma 17-70 you need to check that sharpness in all 4 corners of the image will be uniform... it seems that many Sigmas (17-70s) have some optical elements tilted within the barrel.
08-21-2008, 08:21 AM   #3
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try some brick wall tests and check the corners
usually for a zoom, the extreme corners at the ends of the zoom wide and long are the worst
08-21-2008, 08:23 AM   #4
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the corners look quite sharp, but looking the dome you can see purple ang green lines on the borders...

08-21-2008, 08:31 AM   #5
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The seller is from a friendly pentaxian forum in Italy, he usually sells optics, he told me spent 50 euros (75USD) to adjust the optic and now it is perfect. He looks honest, he has to mantain his good reputation so I think he won't sell rubbish... I would like to change my 18-55 AL II (wich is pretty good) and I won't have a worst optic...
I just would like to know why these colours appeared on the picture and if it is normal.
(Sorry 4 my English...)
Thanks 4 your help!!
08-21-2008, 08:33 AM   #6
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QuoteOriginally posted by pucce84 Quote
the corners look quite sharp
did you check the brick wall/newspaper shots ? if not - do.
08-21-2008, 08:50 AM   #7
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The top photo looks like a winner, so I'd ask him for a few more photographs to help me decide.

If possible tell him what you'd like to see in the photographs.
08-21-2008, 09:21 AM   #8
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You call that abberations?? Man, they're barelly visible!

08-21-2008, 09:27 AM   #9
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QuoteOriginally posted by Spex Quote
You call that abberations?? Man, they're barelly visible!
That's my opinion as well.
08-21-2008, 09:41 AM   #10
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QuoteOriginally posted by Spex Quote
You call that abberations?? Man, they're barelly visible!
I tend to agree. Just a pixel or two wide viewed at 100%, shooting directly against the sky on a bright day at the extreme widest angle. Not sure what aperture it was at. If f/8, that *might* give me slight pause - but I'd probably want to compare it agaisnt the kit lens in exactly the same situation to see. But if its wider open than that, or stopped down to the point of diffraction and that's all the CA you see, I personally wouldn't be disappointed.
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