Should I sell my 17-40 f/4L for the 17-55 f/2.8 IS?

[neuroanatomist]

Thanks for the link. It looks like 17-55 is sharper at most apertures except at f/11, which is sharper for the 17-40 at the centre. I mostly shoot at f/8 though, and it looks like 17-55 has a very slight edge in this area...

No, the 17-55mm has a *big* edge. You're comparing the 17-55mm on a 50D (APS-C) with the 17-40mm on a 1DsIII (FF). The FF should be the clear winner (you can see the difference if you compare both bodies with the same lens, try the 200/2L or 200/2.8L). The fact that they're even close in the comparison tool means the 17-55mm is much better on APS-C.

[2484Stryker]

No, the 17-55mm has a *big* edge. You're comparing the 17-55mm on a 50D (APS-C) with the 17-40mm on a 1DsIII (FF). The FF should be the clear winner (you can see the difference if you compare both bodies with the same lens, try the 200/2L or 200/2.8L). The fact that they're even close in the comparison tool means the 17-55mm is much better on APS-C.

When I compared the two lenses, I changed both to 50D. Are you saying that the 17-40 was only tested on the FF and so even when I selected 50D it still showed me the same images from the FF?