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Is it just me or is someone expecting perfection from a so-so zoom lens on a hot day with a CP?
I'm pretty sure it's just you. Especially since that particular photo was taken on a cold February morning. I'm no meteorologist, but I'm almost certain that a heat haze is unlikely under a 50-degree weather.
I think the background softness in A region is due to lens softness. The plants are much closer, and so the effect is much reduced.
I don't believe lens softness would explain the discrepancy between foreground and background sharpness. I think depth of field would be the reason for that, not lens sharpness. Shooting at f/16 instead of f/11 probably would've helped.
The Image wrote:
You might want to send your 17-40 to Canon Factory Service Center to have the specs checked and possibly calibrated.
I don't believe my lens is not suffering from any kind of decentered focus --- I've already provided examples proving otherwise. Foliage on the left side wouldn't have remained sharp if that was the case. One of the first things I do after buying a lens is a test for asymmetrical focus, and so I know my 17-40L doesn't suffer from it.
I'm convinced now that the first explanation offered in this thread (by skibum5) is also the most accurate. I always thought that field curvature would only be noticeable at larger apertures. I never considered it would still be visible at f/11 exposures. But today, I was reading the Photozone review of the EF 24-70mm f2.8... and sure enough, they were demonstrating the exact same problem I experienced with the 17-40L! Shot with settings (24mm f/10) nearly identical to the samples I provided above. His samples demonstrate the effects of field curvature at infinity focus, whereas my samples demonstrate field curvature at close focus (approx 2m). Excerpt from the Photozone Review of the EF 24-70mm f2.8:
The scene below has been taken at 24mm f/10. Now at f/10 you'd normally just rely on the AF which suggests a near-infinity focus distance here. You'd waste some depth-of-field by this but normally everything would still be sharp at this setting (based on this kind of scene). The result can be seen BELOW the sample scene. The extreme corners are blurry - despite f/10 - whereas the lower center is perfectly sharp.
Notice that the sharpness of the bottom center and bottom corner are identical at one focus distance (2m) but not at a different focus distance (infinity). I think that a lot of times, people attribute poor corners to lens softness when in fact the real culprit is field curvature. I've shot my 17-40L countless times at f/11, and have seen how sharp the edges and corners can get. But then once in a while I get strangely soft results like the ones provided in the sample above. Now I know the problem is with field curvature, which is a design compromise (not a lens defect that can be fixed).