+1 for Vision Center.
"Wow--I am surprised no one mentioned (unless i missed it) Vision Center (just google and you will get directions.
There were overwhelming recommended to me many years ago when I first moved to CM from bkk--they are, as i was told, excellent."
Same here - in 2018 or '19 it was this forum's most recommended shop for glasses, and for vision prescriptions which are done by the owner at no charge if one purchases glasses.
I had a prescription from an optometrist at CM Ram but the owner was pretty insistent on wanting to doublecheck by doing it himself (at no charge).
I paid more than I expected then - I think about 3,600 THB.
Would have been cheaper but the most comfortable frames were a name brand.
Note that the shop has no sign in English.
It's at 18/2 Ratvithi Rd, Si Phum Sub-district, Mueang Chiang Mai District, Chiang Mai 50200.
(UN Irish Pub is 24/1 Ratvithi Rd).