I've been to opticians in the UK where they have a machine that will do that. I remember once i'd bought cheap glasses online, then a year or so later went for

I've been to opticians in the UK where they have a machine that will do that. I remember once i'd bought cheap glasses online, then a year or so later went for an eye test; my prescription hadn't changed, but they measured the glasses, and found that they weren't quite right for the prescription, so i got a new pair. An effective sales technique, and assuming that they weren't lying to me, a useful service for me.