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I often use bi-telecentric lenses at work, which means both object-side and image-side chief rays are parallel. However, as I don't really have an optics background, I'm a bit baffled at how this might work. I have an OK understanding of how a single thick lens behaves under paraxial approximation, but telecentricity just seems a bit magical.

Is it even possible to design a telecentric lens system with gaussian optics only? What would it look like? I'm primarily interested in object-side telecentricity, but bi-telecentricity would be ideal, of course.

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A lens is telecentric if the stop appears to be at infinity. It is possible to make the stop appear to be at infinity for both the object side and the image side. The picture below shows a simple example of this: two positive lenses with the stop in between them. The chief rays go through the center of the stop.

The stop is one focal distance behind the first element, and one focal distance in front of the second element. The picture is idealized in the sense that it uses only two paraxial lenses, but it shows the basic idea. In a real lens, there would be multiple elements to control aberrations and keep the overall length manageable.

You can find more detailed examples by a search of the patent literature.

Simple paraxial system, telecentric in object space and image space.

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