What makes a bad copy?
Henry Cooksey wrote:
We're always hearing about "bad copies" of lenses, but what is it
that causes one set of (mostlY) machine cut/polished elements to be
so vastly different to another set, and how the sharpness can vary so
Optical surfaces have to be accurate within sub-wavelength (a few millionths of an inch), and aligned with such tight tolerances. There are quite a few misses in the manufacturing process, which require a redo or a complete scrap. Why they are not caught in the final test / QA stage - that is a different question; here it is the same economic principle that leads airlines to overbook flights.