Hi Baran

A prism with flat surfaces behind an objective should in principle not change the EFL. It will change the EFL however, if you put it somewhere within your objective. Can you make a simple drawing?

Regrads, Dusan

Hi Dusan,

I'm sharing an example file. In this configuration, the prism is placed after the objective lenses, and the system focuses after the prism.

Sorry Baran, but my version of OS doesn’t have a CGDM 2023 glass catalog and I also don’t have your STEP file (prism) among my objects. In this way I cannot help you farther...

Hi Dusan, I'm resending it with the necessary corrections and the updated file attached.

Hi Baran
First, I thought the problem is that meridional rays hit the Schmidt prism exactly on the roof edge. I decentered the prism in X direction, but it didn’t help.

Then I switched from your STEP file to Polygon Objects/Schmidt.POB. Schmidt.POB in Objects Folder had to be redesigned to obtain a roof. At the end I obtained EFL = 0.10mm, which is still wrong.

Sorry, no idea what is going on...

​@Baran.4,

I took a look at your model.  The lens system by itself, without the prism in place, has an EFL = 310.227 mm.  If the prism is replaced by a glass block with the same equivalent thickness, the EFL remains unchanged as expected.  The glass block has only flat surfaces and follows the lens system, so as ​@Dusan indicated above, the EFL will not change.  The location of the focal plane changes relative to the lens assembly, but the image-space principal plane also moves by the same amount, so the EFL remains constant.

When the actual prism is included, then conventional paraxial analysis can no longer be used to find the principal planes, so the EFL becomes undefined or indeterminate.  Zemax doesn’t return an error, but the reported EFL is clearly erroneous.

Here is a screenshot of your model with the three configurations shown.

Regards,

Jeff

Hi Baran

I agree with Jeff’s explanation.

@Jeff:

Jeff would you be so kind as to help me with my “OS freezes while saving file” question from few days ago? There was no response on that until now…

Thank you very much, Dusan