I have a 50D and 40D plus two EF f/2.8 lenses (70-200mm and 17-55mm) that I have taken thousands of photos with (from college football to indoor ballet). The past two years the quality of the image has softened and I am unable to get a high quality photo. I alway use the canon Digital Photo Professional to edit all my RAW images. What can I do to get the quality of the photo back to the originally camera/lense out of the box capability???
It may be wear to the internal parts but it could also be the result of using filters IF you have them installed to protect the front element.IF you have filters on the lenses REMOVE them & test for better results. If you don't have filters the best advice I have to isolate whether it's a lens problem will be to mount your camera & each lens on a tripod & try using AF for a few test shots (move the lens out of focus after each shot so the AF will have to do it's thing for each shot) and then try a few shots using manual focus, preferable tethered to a computer in live view to see if you get a better result.
If you get a better result in MF mode then you'll need to see whether the problem has more to do with the lens or the body, which will require having at least another lens that nails the focus in AF mode consistently using the same test. If the body has the issue the lens won't be consistent (but may be close to in focus if it has a smaller max aperture). If you have a friend with a Canon body that seems to be working properly re run the tests using it too because a lens problem will go to whatever body the lens gets mounted on.
I had a similar experience with a 24-105/4L IS lens. I ended up sending it in to Canon's repair facility in NJ thinking that the IS was messed up since I would get soft images (even blurry sometimes) at shutter speeds that were high enough to rule out handheld camera shake. It came back to me with a repair tag saying they had adjusted the front, back, and tilt focus of the lens and that it was back in factory tolerances. I don't remember the exact price but I think I was charged nearly $200 for the service but that's cheaper than a new lens...