I bought my Sigma 17-50 EX OS 2.8 as an upgrade from my Canon 18-55 IS II in the summer.
Knowing that some people have had problems with getting a good Sigma lens I took lots of test photos, when I received it, particularly at F2.8.
Apart from needing +5 micro adjustment at 17mm and -5 at 50mm to get maximum sharpness on my 7D, the focus was accurate and it gave sharp results.
After using it for a while I became concerned that in some photos the right hand side gave soft results, but it wasn't consistent. I wasn't able to test it further as I moved house and country in September.
I have taken some great photos with this lens in my new locality, but I am still seeing softness in the right hand side in some photos.
Initially I put it down to depth of field, but then I was aware that it only seemed to be a problem when the right hand side of the lens was furthest away from the subject.
I took some comparison photos today with my Canon 18-55 IS. When the right hand side of the Sigma lens is nearest the subject, the photos are sharp from corner to corner. When the right hand side is further away from the subject the right hand edge is very soft and distorted.
Is my lens decentred? See photos below.
Test image.
Extreme right hand side
Canon 18-55 @ 30mm f5, Sigma 17-50 @ 30mm f5, Sigma 17-50 @ 30mm f8
At f8 the Sigma edge improves but it is not as good as the Canon at f5.
Left hand side
Canon 18-55 @ 30mm f5, Sigma 17-50 @ 30mm f5, Sigma 17-50 @ 30mm f8
Here the Sigma is noticably sharper.
Another test image
Extreme right hand side
Canon 18-55 @ 23mm f5, Sigma 17-50 @ 23mm f5
Again the Canon is much sharper at the right hand side.
Left hand side
Not much difference.
Test image with right hand side nearest the lens.
Extreme left and right edges Sigma 17-50 OS @ 28mm f8
Sharp across the photo.
Any help would be appreciated in identifying this problem, but I think it will have to go into Sigma for calibration as something isn't right with it.
Thanks,
Tim
