Ever found your photos soft and fuzzy? Chances are your lens could be forward or back focusing. Not all lenses are created equal, and with different tolerances for different price points, it is likely you lens is miss focusing. Ever checked?
A Lens Calibration Chart is essentially a focus target with a ruler setup which you can use to measure the in focus areas.
If the numbers behind the target are sharper than those in front then your lens is back focusing (focusing behind your target), meaning you’ll need to add some negative compensation to your auto focus. However if it is the other way around then your lens is forward focusing and will need dialing forwards with some positive compensation.
To use you need to have your camera level and square to the target, and you focus on the smaller circle on the right of the chart. This gives you a good bead on your central AF accuracy. You can then use different focus points on different areas of the target if you wish.
Most higher end cameras allow you to dial in focus compensation, and will store the lens information along with the compensation value.
Below is a free template to download. Print onto A4 paper and stick to some foam core, then cut out and hold the ruler on with 2 pins (or nails).