I've always followed the normal route--I have employer provided vision insurance, I would get myself tested every two years by a optometrist that my insurance covered, and I would buy glass using the prescription from the optometrist or a glasses store supported by my insurance. I don't have anything special in my glasses--single focal, no tinting or light sensitivity, and my glasses weren't remotely stylish, and I'd…
Just have the user take a selfie while holding some object of known dimensions on their forehead. Like a dollar bill. Find the eyes and pupils (OpenCV can do that), find the reference object, calculate.
Then take a picture of the prescription, OCR to the numbers (which will need a really good OCR program given the handwriting of many ophthalmologists), confirm the data with the user, and take the order for new glasses. Composite a selfie with pictures of glasses so you can see them on your face.
Order from China, deliver via E-Packet, profit. Who needs Luxotica?