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I am trying to use the 16mm CGL Telephoto Lens for Raspberry High Quality Camera for a microscope project and I am confused by its back focal length being 17.53 i.e longer than the 16mm focal length specification.

The same page shows 16, 25, 35, 50 mm lenses with the same 17.53mm back focal length and a 6mm lens with 7.53 BFL - again longer than 6mm.

Is the focal length of these lenses really constant at the near and far settings?

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  • $\begingroup$ Welcome to Physics. Since this is more about a specific device rather than the principles of optics, this question isn't well-suited to this site. Consider posting it to Photography instead. $\endgroup$ Jan 7 at 18:59
  • $\begingroup$ This is a physics question. It is about the definition of focal length. See Understanding Optical Lens Geometries for an explanation. $\endgroup$
    – mmesser314
    Jan 7 at 20:16
  • $\begingroup$ @MichaelSeifert did search the photography section for focal length related questions - but I predominantly found optics and lenses related posts in physics section. $\endgroup$
    – sith
    Jan 8 at 20:02
  • $\begingroup$ @mmesser314 - thanks for the link. I did review that link and a few others before posting this question - in all their illustrations EFL is always longer than BFL. $\endgroup$
    – sith
    Jan 8 at 20:04
  • $\begingroup$ If you change your mind and want to migrate this question, raise a custom flag. You might get interesting and complementary answers on both sites. If you decide to cross-post, it's polite to link the two questions. $\endgroup$
    – rob
    Jan 8 at 22:52

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