questions about 1.74 index

[JimGreer]

Can anyone help me? I am trying to determine how the Rx is compensated by the manufacturer (such as Varilux) to determine check out power. The Rx is usually changed by a small amount (.15D, etc.) from the prescribed power so we can check it out correctly. The axis is also changed more often than not. We are starting to field questions from the accounts thinking the lenses are off power from what they request. How can I explain what is taking place and why it is necassary to do so? Is there any info we can send out with the jobs to educate the accounts? Any insight is appreciated.

Jim Greer
Jorgenson Optical Supply

[Darryl Meister]

What you're referring to is known as as-worn optimization. When a lens is tilted, it introduces an increase in sphere power and cylinder power at axis 180 (as a form of oblique astigmatism). Although the effects are negligible in low-powered lenses, tilted high-powered lenses may require you to adjust the actual prescription slightly in order to produce the desired prescription for the wearer once the lenses are in their "as-worn" position.

Essentially, this adjustment reverses the optical effects of the tilt. The adjusted -- or "compensated" -- prescription is the prescription you should verify in a focimeter. A lens that produces the correct compensated Rx in a focimeter will produce the correct prescribed Rx for the wearer once the lenses are worn. You can get more information in the As-worn topic in the progressive lens course at
OptiCampus - Optics of Progressive Lenses.