Why are my Sigma 24-70mm f/2.8 portraits still soft on a Canon 40D?
Asked 11/2/2011
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I’m using a Sigma 24-70mm f/2.8 on a Canon 40D. Before a firmware update, most shots were soft; the update helped a lot, but I still get soft images, especially for portraits. It seems worse at longer subject distances, and low light can still be problematic even with flash. Exposure and white balance look fine, and another photographer also saw the same softness with this lens/body combination. What is the most likely cause, and what should I do next?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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I had the Sigma 24-70 f/2.8 HSM and had all kinds of problems focusing with it. I tried to use it on a Canon 7D and 5D mk II but found much the same problems existed on both bodies. I had problems similar to yours in that there was an inconsistent focal error over the zoom range such that if I corrected for 24 (I don't think you can do this on the 40D) it would give soft focus above 40 and if the subject distance changed this also threw things out. In the end I sent it back and got another copy which unfortunately had much the same problem. After getting the second copy looked at by Sigma the problem was slightly reduced but still not completely resolved.
It's not that the lens cannot focus because I could get pin sharp focus in manual mode but I think it may be a general problem with this lens and autofocus as I have seen lots of forum posts describing similar experiences. In the end I got the supplier to agree that the lens was defective and got a refund. Ended up stumping up the extra cash for a Canon 24-70 f2.8 L which auto-focused perfectly and gave noticeably better image quality but at a pretty hefty extra price.
Originally by user6603. Source · Licensed CC BY-SA 4.0
user6603
14y ago
0
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This sounds more like a focus calibration or lens-copy issue than a general exposure problem. From the reports here, some Sigma 24-70mm f/2.8 copies can show inconsistent autofocus errors across the zoom range and at different subject distances, which can make portraits look soft even after a firmware update.
A good next step is to test systematically: shoot a focus chart or another flat, detailed subject to see whether focus is consistently front/back focusing or if one side of the frame is softer than the other (possible decentering). If manual focus gives sharp results but autofocus does not, that further points to AF calibration.
Because the Canon 40D does not offer AF microadjustment, the practical fix is to send the lens to Sigma for adjustment or replace it under warranty if possible. If a replacement copy behaves the same way, have Sigma inspect both the lens and body together if they offer that service.
In short: document the issue with test shots, then pursue service or replacement rather than relying on post-processing.
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