As someone with fairly pronouced Astigmatism in the expensive glasses camp, I'd say:
1) The glasses have to accurately hold my the lens at a specific location and orientation relative to my eyes in order that they actually work.
Off by a little bit and it's world of splitting headaches and crazy distortion
Achieving that positioning, while dealing with my wonky ears, size of head, eye locations etc - requires knowledge beyond current web systems before ordering and - unless completely bespoke fabrication of frames as well - some form of final fitting post delivery.
- The prescription is sufficiently rare that the lens are not mass produced and instead have to be custom ground and cut to fit the frames before the glasses can be assembled.
1) The glasses have to accurately hold my the lens at a specific location and orientation relative to my eyes in order that they actually work.
Off by a little bit and it's world of splitting headaches and crazy distortion
Achieving that positioning, while dealing with my wonky ears, size of head, eye locations etc - requires knowledge beyond current web systems before ordering and - unless completely bespoke fabrication of frames as well - some form of final fitting post delivery.
- The prescription is sufficiently rare that the lens are not mass produced and instead have to be custom ground and cut to fit the frames before the glasses can be assembled.