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Lens to lens distance is not 2f in "4f system"

  1. Jul 2, 2014 #1
    I studied 4f system and I've got a question.
    As far as I know, 4f system is optical filtering system (attached file).

    In the configuration in the attached file, region between L1~L2 is infinity optics(ray components from P1 is parallel with each other) so that distance between L1~L2 shouldn't affect the image at P3 as far as the distance between L1~P2 is 'f', in my guess.

    Am I correct? Is there any difference between "1), 2)" below?

    1) P1~L1=f, L1~P2=f, P2~L2=f, L2~P3=f (standard 4f system)
    2) P1~L1=f, L1~P2=f, P2~L2=not f, L2~P3=f

    Thank you in advance.


    Attached Files:

  2. jcsd
  3. Jul 3, 2014 #2


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    Science Advisor
    Gold Member

    Configuration (2) will work as long as L2 is seeing Fraunhofer diffraction, and the lens is collecting sufficient light to form an image at P3.

    I usually use a small CCD camera at P3, with the optics removed. The final distance is then adjusted so that the image fills the CCD chip. Then the position of the L2/P3 are jiggled a bit to bring the image into focus.
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