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I bought a bi-telecentric lens for experiment. From all I've learned before, it accepts only axis-parelle incoming beams and exits the same parelle light, so it should be an afocal system with relatively constant imaging performance regardless of working distance. But it's not. It blurs image when nearer or further from a fixed working distance, just like a regular lens. Why is that? I've looked up many explanatory articles and descriptive drawings, still no clue. Customer service man is of no help, unfortunately. Really appreciate any help from you guys!

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A telecentric lens is not necessarily afocal.

An a focal system is a system which accepts light from infinity (collimated light) and 'sends' it to infinity at the output (image is at infinity). The most typical example is a laser beam expander.

a telecentric lens is a lens where the aperture coincides with the back\front focal plane of the lens (or both, in the case of bi-telecentric lens), but this does not mean that the system is afocal, rather it means that all chief-rays (or light cones) are parallel to the optical axis (the image below should help illustrate). It also means that the pupil of the system is at infinity, but this is less intuitive. An added benefit of this fact is that for object-space telecentric lenses, perspective error (magnification vs. distance) are reduced in the resulting image.

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enter image description here

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