This focus calibration chart can help you test the accuracy of your SLR camera's autofocus, and find the right focus micro-adjust setting if your camera has that feature.
It's designed to be printed out and stuck onto poster-board, to make an accurate free-standing focus target.
The principle is simple: you set up your camera level and square-on to the focus chart, take a picture, and then use the distance scale to assess how far in front or behind the target your camera actually focused. For reliable results it's best to take several photos, manually resetting the focus to infinity between each one. The recommended distance between the camera and the focus chart is around 50 times the focal length of the lens.
Footnote: the version in the PDF is slightly improved from the one shown above. I've increased the number of contrast lines in the centre of the target based on some info I read from Canon.
Tim Jackson's focus test chart is a model of simplicity. You just print it out on a single piece of paper, lay it on a table and photograph it from a 45° angle.
Unfortunately the original URL for it, http://focustestchart.com/chart.html, no longer works, but if you may still be able to find a copy via Google.
Here's an excellent article by Keith Cooper that covers AF microadjustment in considerable detail, including a clever method of focus checking developed by Bart van der Wolf, using Moiré patterns on an LCD screen:
www.northlight-images.co.uk/article_pages/cameras/1ds3_af_micoadjustment.html
Invisicord.com provides a chart that is similar in principle to mine, but it's designed for simple assembly by just cutting and folding, no need to stick it to poster-board:
photography-on-the.net/forum/showthread.php?t=750736
www.invisicord.com/docs/invisicordfocuschart.pdf
There are a couple of commercial products that also work on a similar basis to mine: the SpyderLensCal from Datacolor, and the LensAlign Mk II from Michael Tapes Design.
If you want to link to my focus chart, please link to this page; don't "hot-link" directly to the PDF file.
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