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

[lioiviei14]

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.

 

[UltrafastPED]

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.