I noticed that the effective focal length can be negative for a relay systeem with a real object distance and real image distance.
Based on the lens formula you suspect then a positive focal length.
for f1=80mm, f2=45mm and d=235mm gives an EFL=-32.7m, this is simmular to zemax.
My question is: Is the EFL really negative and how am I supposed to interpret this?
Here it means that the image will be inverted.
Hello,
By definition the Effective Focal Length is the distance from the rear principal plane to the paraxial focal plane. A negative focal length means that the rear principle plane is behind image surface. This is due the intermediate focus, which flips the rays top to bottom.
Best,
Csilla
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