Something I do from time to time is to test the autofocus on my various camera/lens combinations but luckily so far I haven't had to make any adjustments. There are lots of charts available to download from the web and print out but my favourite is this image that's designed to be photographed from a laptop screen. It uses moirĂ© fringe techniques to produce interference patterns with the camera's LCD, but looking at it at full resolution without a camera in the way produces other fascinating patterns, which is another good test, for my brain's autofocus system.