Thin lenses and freeform.

**drk** · 05-06-2021, 12:27 PM

I have only a vague notion of this, so help me ask the question, and then answer it, too. :)

Zeiss says that their designs are intentionally thinner. I don't know how they do that. It has been discussed it has to do with freeform eliminating the need for the optical center (an old term/old concept) to be up near the pupil, and such that it can be placed lower in the frame (like, 1/2 B) in order to maximize thinness.

Question: why wouldn't every lens manufacturer do that? Is this a special feature for Zeiss designs, or just any freeform design? Are they just touting the improvement over molded?

**Lensman11** · 05-06-2021, 12:43 PM

Where in the Zeiss literature do they make this claim. Prism thinning has been added to progressive lenses for decades. The purpose of prism thinning is to equalize the thickness on the top and bottom of the lens so the entire lens can be made thinner. Without prism thinning the lens would thinner at the bottom and thicker at the top. This in reality lowers the o.c. of the of the lens. If done correctly the amount of prism thinning changes with the shape of the frame and the fitting height yielding the thinnest lens possible. In the early years before the sophisticated LMS computers prism thinning was just based on the add power, it helped reduce thickness but was not as effective as the it done calculated today.