This is similar to Rod Wodaski's diffraction spike focusing.
Rods is much easier, and just as accurate as it uses the same method (Diffraction spikes). An example here
http://home.comcast.net/~dandkmccrad.../FocusMask.htm can be seen.
No measurements, photocopying etc, so much easier.
Theres plenty more ways to use diffraction spikes for focussing, just need to look. As was found here by Dennis.
Theo