Having just recently studied this, I can say that "equithinning" up to 2/3 of the strength of the addition (but not exceeding 3 D total prism) isn't expected to affect the patient in any negative way as long as both lenses carry an equal amount of prism. Practically speaking, I can't say that I can recall an incident where someone's adaptation was affected by yoked vertical prism, as long as it was equal. I've seen re-do's where the replacement lens has come in with different prism thinning than it's mate, though those
usually don't make it out of our lab.
I can think of at least a couple specific patients that have pretty finicky tolerances who don't seem to be affected by varying amounts of yoked prism, though in all honesty I hadn't considered it in that light until now. (we have a handful of patients who seem to always need 2 or 3 re-makes with every order, for one reason or another...)
The main benefit is a thinner, lighter lens, and "my new glasses are too heavy" is probably one of our most common complaints so I can't see any reason not to do it.
As a matter of fact, it's not even one of the parameters we specify from any of our suppliers: they all just do it automatically. With the result that I've never dealt with progressives that weren't prism-thinned.
Edit: now that I'm thinking about it, I've just realized that I've recently seen prism thinning that exceeded 2/3 thirds of the add... IIRC it was 2.13 D prism with a +2.50 add...
and it was one of those remakes that didn't get caught before the patient found it for us
Bookmarks