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Keeps referring to high-NA and low-NA. What's NA? (Numerical aperture is all that comes to mind.)

In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.

https://en.wikipedia.org/wiki/Numerical_aperture

Numerical aperture

https://en.wikipedia.org/wiki/Numerical_aperture

Heh, edit jinx. This answer leaves me almost as at sea, so I guess my time's not best spent on this article. Thanks.

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