Wrap compensation = Axis change?

[DC Optix]

I need a little help understanding something here...

I frequently order wrap compensated PAL designs for Rx suns. I totally understand that the sphere and cylinder powers will change due to the wrap, but I had one the other day that the axis changed 20 degrees in each eye. I can't seem to grasp why this is the case.

Could somebody shed some light on this for me? I really appreciate it!

[shanbaum]

Tilting a lens induces a change in sphere power and also induces cylinder along the axis about which the lens is tilted (see, Martin's formula). If the original cylinder axis was at some meridian other than the axis about which the lens is tilted, the induced cylinder will shift the original axis, as the induced power and original power are combined (as when doing an over-refraction).

[Wes]

A minus lens will induce minus cyl at the axis of rotation and a plus lens will induce PLUS CYL at the axis of rotation. Thus a minus sphere wrap will induce minus cyl at axis 90, so the compensated rx will have cyl at axis 180. A plus sphere wrap will induce plus cyl at axis 90, transposed to minuc cyl form into induced cyl at axis 180 so the compensated rx for plus sphere lenses will have minus cyl at axis 90, the opposite of minus sphere lenses.

It gets more complicated when you add cyl into the equations. To add to that, the higher the power of the lens, the more cyl it will induce. So if you have a high sphere power, and a low cyl power, the compensation could throw your axis off by a very large degree. I've seen -.25 cyls have the compensated axis changed by more than a hundred degrees, and increased by a half diopter.

[QDO1]

by tilting a lens, you effectively add a cylinder (plus or minus). So if you order a wrapped (tilted lens) you need to adjust the cylinder (and usually the axis and sphere) to get the same vergence as the RX at the eye