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. The marks and text are small and gray to avoid the camera focusing on them instead of the high contrast mark.

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: