Hello everyone,
I have an optical design with an effective focal length of 250 mm. However, after adding a Schmidt prism, the effective focal length becomes -0.106 mm, whereas it should be around -205 mm. Could you please help me with this issue?
The prism changing the effective focal length.
Best answer by Jeff.Wilde
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
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
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