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-------------------------------- Optics Primer - Teleconverters

--------------------------------------------------------- By Bill Claff

When we placed a positive power lens behind our main lens we had a reduction in focal length.
By putting a negative power lens behind the main lens we can increase focal length; a teleconverter (TC).

All the following material is from US patent 5,253,112. Here is the main 300mm f/2.9 lens presented there:

The patent states this is a 294mm lens and the Optical Bench reports 293.60mm.

The teleconverter looks like this:

Note that the rays diverge rather than converge; this lens has a negative focal length (power).
The patent states -97.597mm and the Optical Bench reports -97.15mm.
Also, the height at the image plane is twice that of the incoming parallel rays; verifying that this is a 2x teleconverter.

Together it looks like this:

You can immediately see the reduced angle of view.
The Optical Bench reports 8.37 degrees without the teleconverter and 4.19 degrees with; this is exactly as expected for a 2x teleconverter.
According to the Optical Bench the focal length and f-number don't exactly follow the 2x we would expect.
Focal length went from 293.60mm to 579.43mm and f-number from 2.90 to 5.72; both are 1.97x changes rather than exactly 2x.
This could be the Optical Bench but a more likely explanation is that the entrance and exit pupils don't coincide with the front and rear principal planes.