Mask Patern for Focus

6 kinds of mask paterns were here and calculated of diffractional patern near focal point to know which one is best to find the focal point.
6 mask paterns, two holes, three holes, four holes, two squares, four squares, and cross.

First of all, let's see those diffraction paterns at the focal point and 180 and 360 micron meters step away foward and back of the point. Optics' Fno is 10.0.

It is easy to understand that holes' and square paterns are not sensitive for defocus, which means that they are not suitable to find the focal point. The cross patern's diffraction changes dramatically by defocus, so this would be best mask for finind the focal point.
Inspect more carefully. Next diffractional paterns are at the fucal point and 90 and 180 micron meter foward and back of the point.

Next image was calculated with 9 micron meter squares pixles, and defocus was 90 micron steps, half of focal depth.

Three images at center are in focal depth, 180 micron meters. The cross diffractional paterns are very sharp, and out of focal depth is blur.

2000, January 26th, original site in Japanese
2002, June 2nd, in English

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