Simple and accurate method for determining lens focal length

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


A simple and accurate method for determining the focal length of a lens is presented. There is no need to know the distance between lens principal planes and can be performed with easily available and inexpensive hardware. Depending on the test lens (TL) f-number, the error on determining focal length can be in the order of 0.1% of the focal length. We discuss determining focal length with the Newton equation f2 = zz ′ , with the Bessel equation f = ((L - PP)2 - D2) / 4 (L - PP), with equation f2 = y ′ (y + y ′ ), or with equation f = (y2 - PP2) / 4PP.

Original languageEnglish (US)
Pages (from-to)34108
Number of pages1
JournalOptical Engineering
Issue number3
StatePublished - Mar 1 2023


  • autocollimation
  • focal length
  • focus measurement
  • optical bench

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics
  • General Engineering


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