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04-30-2008, 07:30 PM   #1
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Need help to check lens. Pictures attached.

I have Sigma 17-70mm lens that I have purchased new from authorized on-line dealer. I took few pictures of the side of my building today to try to determine if the lens has equal focus quality. Unfortunately, after looking at the pictures for 20 minutes I came to conclusion that I either do not know what I was supposed to look for or the pictures are OK. I know that at f/2.8 the edges will be and at f/8 or f/22 all appears to be in focus (at least to me).

So help me please check the lens. The pictures were originally shot as RAW and converted to JPG and resized in iPhoto. No other PP was done. EXIF should be intact. There might be some blur at f/22 due to the handheld camera with shutter speed being 1/60 and 1/25

Here are the pictures:

17mm, f/2.8
17mm, f/8
17mm, f/22

35mm, f/3.5
35mm, f/8
35mm, f/22

70mm, f/4.5
70mm, f/8
70mm, f/22

Thanks

05-01-2008, 12:03 PM   #2
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Afternoon bump.

Anybody?
05-01-2008, 12:18 PM   #3
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I'm never shot brick walls before but plenty of various store shelves at the camera store to verify 'sharpness', CA/PF and other things back when my LBA was in full swing.

I looked at your photos and they seem pretty good. The 17mm F2.8 shot ...doesn't look like you centered it well. Probably why one side looks a tad softer that the other.

Overall it looks pretty decent to me. Remember that all lenses do have some field curvature and so you shouldn't use brick walls and newspapers to decide your satisfaction upon. Just go out take some photos of flowers, architecture, landscapes. Much more rewarding! If problems are still bugging ya then post some more photos + the exif and we'll get a better picture of the lens!

Hope it helps!
05-01-2008, 12:23 PM   #4
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I've got the 17-70 and if what I'm seeing is what you're proposing (the gentle waviness), then I think there's something amiss.
Take a look at this image I took on World Pentax Day:



From my vantage, this on looks pretty reasonable @48mm. If your symptoms were common to this lens I would expect to see distortions.

05-02-2008, 06:08 AM   #5
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Thanks guys. I have another pictures that I have taken with the lens. I will go over them and if I spot something, I will surely post again asking for your help.
05-02-2008, 07:29 AM   #6
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I'm with FotoPete

I never found walls good for anything but validating exposure accuracy (where I do use them)

real worls photos are about the best thing to consider

Also as FotoPete noted there is some field curvature in most lenses, especially at wide angles.

If you want a true flat field lens, get a macro (not a zoom with macro capabilities, but a true macro)

That is what they are designed for
05-02-2008, 08:25 AM   #7
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Your photos look good to me. It doesn't help in judging that the brids aren't uniform, so for example they don't have the same texture on the right hand side as the left.

I think you got a good copy

John
05-02-2008, 07:25 PM   #8
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Looks like a case of the telltale moustache distortion (a mix of barrel and pincushion distortion). Probably the most difficult to correct in software. Don't know if this is common to Sigma because my friend's Sigma 10-20mm also exhibits this distortion.

05-02-2008, 07:37 PM   #9
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The images look soft. Try using a tripod.
05-02-2008, 07:45 PM   #10
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Yes, there is mustache distortion. This is common enough in modern zoom lenses. If it is an issue in a particular picture, I would suggest PTLens as a reliable way to do the correction. As for checking for centering defect...the brick wall is not the best target. Photozone.de used to have a Web page detailing how to do a test for consistent performance across the field. I just tried to find it, but was unable to locate it on their new version of the site. It involved shooting a sheet of newsprint at various apertures with strict attention being paid to making sure that the camera detector plane is parallel to the target.

Steve
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