I have recently invested in a lens calibration kit in order to improve the sharpness of my auto-focusing. This consists of a plastic target that mounts to a tripod to lighting stand that you will photograph using the lens to calibrate from your camera mounted from another tripod with the camera set to Aperture Priority, if a zoom lens set to it’s longest focal-length and to it’s widest aperture. Then using the auto-focus you will photograph the target and either looking at the image from the back of the camera or better still from a tethered computer look at the image and determine if the auto-focus needs to be adjusted up or down through the camera menu depending which half of the scale appears more focused. Additional photos are then taken to determine the correct setting once the best setting is found this is remembered in the lenses internal electronics; so each time that lens is refitted to your camera that new setting is automatically recalled. This auto-focus calibration is found in the menu of most if not all new Cannon and Nikon DSLRs, I am using a Nikon D800e with Nikon lenses.