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

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SUMMARY

The discussion centers on the optical filtering system known as the 4f system, specifically addressing the lens-to-lens distance in configurations that deviate from the standard setup. The user, Sungsik, questions whether the distance between lenses L1 and L2 affects the image at P3 when L1 to P2 is equal to the focal length (f). The consensus confirms that configuration (2) can function effectively if L2 observes Fraunhofer diffraction and collects adequate light for imaging at P3. The use of a CCD camera at P3 is recommended for optimal image capture.

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  • Understanding of optical systems, specifically the 4f system
  • Knowledge of lens focal lengths and their implications in imaging
  • Familiarity with Fraunhofer diffraction principles
  • Experience with CCD camera operation in optical setups
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  • Research the principles of Fraunhofer diffraction in optical systems
  • Learn about the design and applications of the 4f optical system
  • Explore techniques for optimizing CCD camera positioning for image clarity
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Optical engineers, physicists, and anyone involved in designing or utilizing optical filtering systems, particularly those working with 4f configurations and CCD imaging.

lioiviei14
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Dear,
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.

Sungsik.
 

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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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