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PostPosted: Thu Apr 09, 2009 4:59 pm 
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I just ordered my Sigma 10-20 and will have it tomorrow;) but wanted to ask how should i test it to make sure it is a good copy?

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PostPosted: Thu Apr 09, 2009 7:13 pm 
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Best test in nature is either grass or bare trees/bushes with fine twigs (what ever you can find in this time of the year). Shoot at 10mm, 14mm, 20mm open wide (f4.0-f5.6). Have enough distance (at least 20m from trees, 10m for grass), that means you need a long row of trees parallel to your sensor or a large/wide lawn. use the centre AF-point only and focus on the canopy of the trees resp. a piece of grass far enough away from you.
Now: switch off the AF (after focussing) because you're going to make two shots and don't want any different AF-adjustment for each shot.
1st shot: tilt the camera approx. 33 degrees to the left so that the horizon would cut through the upper left and the lower right corner of your view-finder. Then aim the centre-AF-spot at the point where you have focussed and shoot.
2nd shot: tilt the camera 33 degrees to the right, so that the horizon runs through the lower left and the upper right corner of your viewfinder, aim at the focus point and shoot.
Now you have 2 photos with sufficiently small structures in the centre an the resp. corners that you can compare. As always centre sharpness should be best. If this is not the case repeat the test-shots. Don't be disappointed if the corner-sharpness is not as good as in the centre but in a well-adjusted lens the corners should show similar sharpness. If one or both corners on one side are clearly less sharp than on the opposite side the lens is decentred - or you didn't watch out that the trees or the grass were parallel to your sensor i.e. have an equal distance in both corners from the camera. In the latter case redo the test-shots.
If you are looking for examples of differences in sharpness between corners or between corner and centre which are not tolerable, looky here and there. I sent both of those lenses back.

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PostPosted: Thu Apr 09, 2009 8:07 pm 
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Ok thanks sure is a list you have there lol, another thing i wanted to ask you on the AF-Points it seems like with all my lenses when i only choose the center AF-point the pictures are more sharper than when you have all the AF-Points chosen why is that? this is why i always keep the center AF-point chosen;)

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PostPosted: Thu Apr 09, 2009 8:17 pm 
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I've posted your Q regarding AF-points in the Canon section here. You should get more hits there.
I also deleted the quote in your last post to make it leaner and easier to read.

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
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PostPosted: Thu Apr 09, 2009 8:25 pm 
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Thanks;)

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PostPosted: Fri Apr 10, 2009 3:45 pm 
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Here is an example of the kind of photo I would use for testing a lens for decentering defects and corner quality. I didn't cut out the crops, but if you click on the images you can get (via "all sizes") to the large originals and judge the corners at 100%. Hint: the upper left corner of this lens (a micro Nikkor 105/2.8 VR) is a little weak but otherwise sharpness looks perfect at f/2.8.
Image Image

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