Have you ever read a “for sale” ad for a lens and have someone talk describe it as a “sharp copy”? I have to admit, the idea that there are good and bad versions of lenses scares me – wouldn’t there be some sort of testing and quality control?
I recently read an article about lens sharpness and sample variation from the owner of Lensrentals.com (if you aren’t familiar with them, they are what their names says. If you’ve ever wanted to try out an exotic lens for a little while, check them out).
The net of the article is this: Even the very best lenses have variation, but so do the bodies they mount on. Some combination of a lens on a particular body might yield great results, but switch to another sample of the same body and things look different. They are all going to vary a little, and that is OK. The point he boils down to with data and analysis is that bad lenses are truly rare, and you shouldn’t waste a lot of time trying to match a lens and a body. It will drive you crazy. Get good, reputable lenses and learn to shoot them as well as you can. Sharpness starts with the shooter.