Many people should be familiar with the idea of pincushion distortion if they’ve ever looked into a curved mirror, similar to being in one of those carnival ‘funhouses.’ If the mirror is concave (like looking at the inside of a spoon, it bends inward and away from the viewer), the...
I'm happy to see you included exceptions to your NEVER comment. I've ground and polished a few mirrors and I tracked the focal length with a spherometer, "wet" mirror solar test (at the focal point) before the glass was polished, and polished mirror "flashlight" test (at the radius o...