Digital method for measuring the focus error

Adolf W. Lohmann, David Mendlovic, Zeev Zalevsky

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


We show how the focus longitudinal error z can be deduced from the observed image. This deduction requires some digital image processing. Once the focus error z is known, one may compensate for it either by shifting the lens or by deblurring the image digitally. We have studied systems with a circular or with a rectangular aperture. In the former case, the defocus causes some dark rings in the frequency spectrum of the image. With a rectangular aperture, the defocus creates a set of straight, orthogonal dark lines. These dark zones provide a quantitative diagnosis of the focus error. Some experimental evidence supports our concept.

Original languageEnglish
Pages (from-to)7204-7209
Number of pages6
JournalApplied Optics
Issue number28
StatePublished - 1 Oct 1997
Externally publishedYes


  • Autofocus
  • Digital image processing
  • Focus
  • Imaging
  • Optical transfer functions


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