Quote:
Originally posted by Darryl Meister:
Hi Pete,
For prescribed prism... I don't believe that vertex distance affects the prismatic effect for distant vision, but I do believe it will affect the prismatic effect for near vision slightly. This has to do with a change in prism "effectivity" at near. If you want to know more, I can post the formula to calculate prism effectivity at near.
For induced prism... As the wearer looks away from the optical center of the lens, prism is induced according to Prentice's rule. As the vertex distance is increased, the eye moves across more of the lens surface for a given angle of view -- that is to say, the line of sight will move farther from the optical center, which will induce more prism, for the same angle of view.
Best regards,
Darryl