On Wednesday, 17 April 2019 12:01:39 UTC+1, NY wrote:

I doubt whether polishing off a few thousandths of an inch from the surface
of the lens which is probably a couple of millimetres thick will have
altered the focal length by a detectable amount.

You've (rather admirably) used inconsistent units of measure - could we have
this expressed in linguini instead?
https://www.theregister.co.uk/Design/page/reg-standards-converter.html