Alex, with your setup, the camera could indeed lock on to different lines at different times. That tape with all the closely spaced lines is just inviting this.


Here is a proper test chart designed to help you lock onto the intended line. It does make a difference. Notice that you can print different charts depending on your printer. I use the faintest chart and always lock on to the center. i tried some of the more dense charts, and AF did not always lock on to the center, so you might need to try different ones.


There is a excellent discussion of things that can go wrong while testing, and a nice section on how to intrepret the results.


I found it to be useful as a starting point, once you understand whats going on, then there are a number of good ways to check autofocus.


regex.info/.../focus-chart