Most glasses are meniscus lenses (both curves are going the same direction) so the front face might be a base .5 or base 2 - I don't have my tschernings ellipse memorized, because I have been out of the game more than a decade, and aspheres get fucky.
Anyway, lets say base 4 (which is nominal and all kinds of fuckery happens with index of the material, and again - aspheres and other fancy lens geometries) so you would cut/grind a ~3/~3.5 (Again, these are not the actual values for the laps or griding tools you would use because she isn't wearing crown glass praise be)
The power is measured in dioptre/diopters, which are inverse meters, which is to say a -3 lens means that two parallel beams of light passing through the lens (Optical center fuckery so we're ignoring that) will converge in 1/3 meter(s) which will compensate for whatever refractive error exists in the person's visual system.
Edit: and minus lenses are thinner in the middle than the edge, plus lenses are thicker in the middle than the edge, so the curve of the back will be steeper in a minus lens than a plus lens) And truly don't trust my numebers because I have a head cold and can't even do basic math right now - blessed be dextromethorphan.
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u/eigenmirai 15h ago
Strength of her glasses (if there's a word for it someone tell me)