DIGITAL METHOD FOR MEASURING THE FOCUS ERROR

We show how the focus longitudinal error z can be deduced from the obs erved image. This deduction requires some digital image processing. On ce the focus error z is known, one may compensate for it either by shi fting the lens or by deblurring the image digitally. We have studied s ystems with a circular or with a rectangular aperture. In the former c ase, the defocus causes some dark rings in the frequency spectrum of t he image. With a rectangular aperture, the defocus creates a set of st raight, orthogonal dark Lines. These dark zones provide a quantitative diagnosis of the focus error. Some experimental evidence supports our concept. (C) 1997 Optical Society of America.