17-40L with massive focal place curvature?

I have a problem with my 17-40L and I’m looking for advice on whether it is worth trying to send it in for repair. It is not under warranty anymore, so wondering if this a ‘known’ and reasonably fixable issue, or something that is going to cost so much that it’s not worth it? It appears to be severe focal plane curvature. I’ll explain what I’m seeing with a couple of examples.
So I used the lens on a crop body for a while and never noticed any problems. However, on my FF I noticed that the corners seemed excessively soft, even at f8-f14, particularly at the long end. Upon further investigation it seems that if I focus at say a tree at 30ft, 40mm, f11, the center and everything to infinity (or at least for 100 yards) in the center is in focus, but the corners are very out of focus. I say OOF, rather than just soft, because if I turn the focus ring towards (or possibly past) infinity the corners come into focus while the center then moves OOF. Of course this is what I’d expect from a slightly curved focal plane but this is very pronounced, particularly when focusing at about 20-50 ft. I have also tested this on a wall at 15ft and get the same result (slightly less obvious in the compiled images)
I suspect this is happening to some degree at 17mm but it’s much harder to see. Perhaps because the corners are so soft at wide apertures and the DOF is large when stopped down? Still I find the corners are very soft at f8 and actually get sharper up to f16.
I will talk to a Canon agent before deciding whether to send it in for repair, but wondered if there was anyone with advice, or had experience something similar, rather than believing everything a Canon person might tell me. I know for instance if it were just a misaligned element (i.e. softer on one side than another), that is easily fixable, but I’ve never heard of a problem like the one I’m experiencing.
The images are compilations and contain a small version of the uncropped image, with rectangles showing the crops. The top pair of crops is from one shot (focused on the tree trunk and wall center respectively) and the bottom pair of crops are the sharpest image obtained in the corner by focusing more towards (or in the case of the park, beyond) infinity.

I's actually slightly worse in reality than is looks in the reduced-size posted images.

Any ideas/comments/knowled​ge appreciated.

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Makes sure you aren't using some cheap filter on the lens. If so, remove it. I think the corner softness is the nature of the lens. Not sure if it is a known issue with the 17-40L on a FF body.

TooManyShots wrote in post #13525125
Makes sure you aren't using some cheap filter on the lens. If so, remove it. I think the corner softness is the nature of the lens. Not sure if it is a known issue with the 17-40L on a FF body.

Yea, good point I forgot to mention that. I did think of this (and was hoping) even though the filters I use are B+W multi-coated because originally I was unhappy with shots I'd taken at 17mm and though I could have a warped filter or something. Actually my testing showed that I could not tell the difference (even pixel-peeping) between no filter, a B&W UV filter and a B&W CLP, even at 17mm.

I realize the corners are soft with this lens, but would you expect them to sharpen with focus adjustment or be sharper at f20 than f11?

ejenner wrote in post #13525220
Yea, good point I forgot to mention that. I did think of this (and was hoping) even though the filters I use are B+W multi-coated because originally I was unhappy with shots I'd taken at 17mm and though I could have a warped filter or something. Actually my testing showed that I could not tell the difference (even pixel-peeping) between no filter, a B&W UV filter and a B&W CLP, even at 17mm.

I realize the corners are soft with this lens, but would you expect them to sharpen with focus adjustment or be sharper at f20 than f11?

Seriously, if you don't pixel peep, you won't have issues. The 5dmarkII is an unforgiving camera because not many lenses can match the resolving power of the senor. The only solution I can see is that you can apply smart sharpening during post processing. Two, get a better lens (16-35L markII or Carl Zeiss 21 or 24L).

TooManyShots wrote in post #13525291
Seriously, if you don't pixel peep, you won't have issues.

Oh no, this is real obvious, even on a computer monitor at full size, especially because the sharpness changes so rapidly for being really decent at the center to mushy at the edge. I don't worry at all about anything I can't see on a 20x30 print and if I can make nice 13x19s I'm happy. The reason I didn't notice it on the crop is that the center is good, the sharpness just dives right about where a 1.6 crop would be.

I don't generally pixel peep too much, which is why I never bothered testing the filters until now.

OK, after some investigation it looks like this is a pretty common problem with this lens and the top left corner, but most people seem to have a problem wider than f8. So because it is so obvious, even at f11 I'm going to sent it in to the Irvine repair center.

I checked the bottom right after thinking about it last night and it it fine if not sharper than I would expect. Top right is not good either, bottom left a bit out, but not terrible.

Anyone had this actually fixed on a 17-40? Or are they just going to tell me it's 'in-spec'?

I had a 17-40 for a day and noticed this issue as well. i took it back since I could not live with it. If I spend that much on a lens it better be a lot better than my kit lens and it was not. I was not the only one who thought that. The 2 people I deal with at sammys looked at the pics and sais the same thing. I did see that pics taken at 17mm or so looked really off and soft. I tried from F4-F11. Nothing seemed to make it better.

You didn't notice the problem on a crop frame because you used mostly the center of the lens elements. A 5DII really shows the weakness of this lens. I ordered a new 17-40 to use on my new 5DII body and returned it for a refund. Really bad chromatic fringing, soft corners, and vignetting with no filter. Although an expensive solution, I got a 16-35 L II and the difference is like night and day. One thing that will help with the 17-40 is to shoot in RAW and use the lens profile tool in ACR. It will correct some of the problems you are seeing. I don't think sending it in to Canon will fix anything.

ejenner wrote in post #13527213
OK, after some investigation it looks like this is a pretty common problem with this lens and the top left corner, but most people seem to have a problem wider than f8. So because it is so obvious, even at f11 I'm going to sent it in to the Irvine repair center.

I checked the bottom right after thinking about it last night and it it fine if not sharper than I would expect. Top right is not good either, bottom left a bit out, but not terrible.

Anyone had this actually fixed on a 17-40? Or are they just going to tell me it's 'in-spec'?

That sounds like a decentering problem more than a field curvature problem.

The 17-40 on FF is soft in the extreme corners from 17-20 mm, but it should be equally soft in the corners, not consistently different as you seem to have.

BTW, this is one of the reasons why I eventually went with a TS-E 17. I compose for the entire frame.

Kind regards, Wim