Help needed - lens decentered, or other potential issue?

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Sjak
Sjak Senior Member • Posts: 6,198
Help needed - lens decentered, or other potential issue?

Hi all!

Just got myself a nice Sigma 35mm Art lens. During the random test-shots, I noticed that the right side of the frame (on flat subject) would be out of focus.

I'd like to have some feedback on the (admittedly improvised) testing I did, and whether these might suggest an issue with the lens.

The lens is under warranty (reputable store) but I want to check if I didnt overlook anything. (If only to avoid an unnecessary trip to the store.)

I've made several test images with the camera being level with the floor shooting the carpet (tripod + water level gage on hotshoe) Manually focused on center of the frame. There is a definite sharpness fall-off at the right side of the frame, and even when stopped down a little. (In earlier loose testing, I've also tried focusing on the right-part of the frame, but then the rest of the image is noticeably out of focus.)

The other series is a try at this test:

https://phillipreeve.net/blog/how-to-check-how-decentered-your-lens-is/

The shots were with an object at maybe 20 meters, from tripod, SR and horizon adjustment off.

So on the fist shot, I manually focused on the center of the image.

Overview, with target marked.

Then, I put this object in every corner of the frame in the next 4 images.

I shot several series, at 1.8 (due to ambient light) and later 1.4. Focused again before each new series. 100% crops, quick exposure adjustment but no other edits. What struck me is that although the distance to the subject should be nearly identical when it's in the right or left corners, in the left corners it shows purple fringing, and in the right corners green fringing. I'd expect the same fringing in all corners, or...?

The 1.4-series:

1.4 - center

1.4 lower left corner

1.4 lower right corner

1.4 upper left corner

1.4 upper right corner

The 1.8 series:

1.8 center

1.8 lower left corner

1.8 lower right corner

1.8 upper left corner

1.8 upper right corner

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