This is a low resolution picture of the test target.

Traditionally to check the focus of a camera a picture of a ruler is taken at an angle of about 45 degrees with the camera focused on, for example, the middle of the ruler. That is not a reliable way to do it since it is unknown what the auto focus selected as the actual focus area. It might have picked a number instead of the desired vertical mark.

To avoid the problem, I created a custom ruler with a large reference mark and very small calibration marks. Further, as a double check, shoot the target from both left and right hand side to make sure everything is consistent.

A last warning: Make sure the target is really flat when checking the calibration. Just hanging it on a wall is no guarantee for flatness.

The file can be downloaded here: