« on: August 22, 2014, 11:27:52 PM »
Testing lenses is a minefield.
First of all, they usually focus differently at different distances, testing at a distance of 50X focal length usually, but not always works best. Its a good idea to test at three or more different distances to see if its a serious issue.
If you can use some flat surface like a brick wall, and make certain you are perpendicular to it, the results can reveal some things like a decentered lens element. The image will be out of focus on one edge while the opposite edge is sharp. Rotate the camera and if the oof edge follows the rotation, you have found a issue. Using star charts can find the issue as well.
I like and use FoCal, but it requires a careful setup, and paying attention to factors that affect your results. Its good to understand those factors that affect the AF system since they apply to normal shooting.
As others have noted, Lens Rentals tests all of their lenses as they come back into stock, and they have learned to make adjustments to many of them. They keep teasing a service for individuals who want their lens tested. However, they are not going to adjust a lens, just test it.
I think that their business is growing so fast that they will not have the capacity to do this.
Canon has a superb facility, and the software to detect lens issues and fix them, but its time consuming and expensive. They do not give you before and after results, and only adjust a lens to be in spec, they do not adjust it to be as good as possible. Some lenses will never be extremely sharp due to the glass manufacturing tolerances, so some are better than others. This is a result of the state of the art. The tolerances involver are far beyond the capability of direct measuring equipment, so indirect methods must be used, and, in the final analysis, the expensive big whites have lens groups selected and graded to get a near perfect match.
Then, there is the lens mounts, which can bend slightly, and the body. Sensors must be adjusted to align perfectly with a lens mount, and be the correct distance away.
I happened across a factory service manual for my old 5D classic, it has the methods and specs for aligning the critical items in the body. Shims are used to tweak things.