Apologies for straying off-topic, but I know there are a lot of clever people here who might know the answer.
A friend of mine wears glasses and has difficulty doing her eye make-up because she can't reach her eyes with the glasses on and can't see her reflection with them off.
She can't readily find a magnifying mirror that's powerful enough, but knows of some expensive mail order ones that claim to be stronger than usual.
I know that if she uses a 3 dipotre magnifying glass on a plain mirror, she can see OK, but the problem is that a mirror will be labelled as 3X or 5X, whereas a lens gets categorised by dioptres or focal length.
Is there a way to convert the preceived magnification as seen through the lens & mirror t( n dioptre ) o get an equivalent rating as defined for mirrors ( n times ) ?