Off Axis???

[Rex]

Has anyone experienced the following phenomenon?
You process an expensive Poly (or Platic) Polarized Progressive with a -0.25 cylinder and the lens comes out 15 degrees off axis, everytime. When you finally say "uncle" and try laying it out to compensate for the problem, the lens comes out as you blocked it. Now 15 degrees in the other direction. UGH!!

The only pattern I've seen to this is low cylinder power progressives with an oblique axis ina "layered" material.

I'd appreciate any input to either solving this or confirmming that I'm not a moron.

Rex " Can you really call a -0.25 cyl prog off axis?"

[Texas Ranger]

Rex, tell me you believe that by the time you throw in the other factors of lens thickness, base curves, vertex distance, tint, polarizing and the fact that the rx axis is basically and educated guess, that any patient could tell the difference in 30 degrees of axis, now 0.50 is different.

[JRS]

Depending on the design, there could be a small amount of cylinder running through the front of the lens. Depending on the axis ground (back side) the appearance of off-axis will show in the lensometer as they might compensate for each other. When you change the axis (again back side) the two may no longer oppose each other and it shows axis as ground. One of the reasons that some lenses appear on axis and some don't. The combination of front to back axis may or may not affect the viewed results. (the front axis usually runs at about 255 degrees as viewed from the front - OD).
If you have a Humphrey you will notice there is a separate mode for PROG. You can see a partial change, by taking a 0.25 cyl PROG (newer design) and checking in both modes. Use the hold down plate so the lens doesn't move and change modes. Watch the axis change some.
On a lensometer take a contact lens stop and place it the lensometer. Again, check the same lens with and without. Also the best area to check a PROG (and I'm talking about the newer designs) is temporal within the checking circle.
The reason the measuring devices show this is because of the size of the viewing circle on lensometers, and the scanning radius on Humphrey.

Obviously, if the actual process was bad to begin with - the above does not apply.

[This message has been edited by JRS (edited 08-11-2000).]

[Jeff Trail]

Hmm.. I'm thinking you may be having a problem somewhere else.. I do tons of polarized (my labs in FL.) The two designs I run mostly are Younger Images and Polar Tech Pro (high index) from Shar and I have not had anything happening like what you are experiencing... are you having this problem with only certian powers? combination with that .25 cyl. ? and is it in anything else other then the polarized ?
Just curious....

Jeff "sometimes those low powers are the biggest pains" Trail

[Rex]

It happens to be most common on plus lenses with an oblique axis. At first I thought it was just people not taking care of the equipment or following the process correctly. Now, I can tell you it doesn't matter who or on what, it happens. Wax & Alloy blockers, radial arm or 3 axis generators and all mixes of cylinder machines 5000, 5056, 5004 even Acquity. I've talked to some wholesalers that take a reading of the raw blank (unwanted cyl) and take that into account. Not using either of the vendors you mentioned. There probably isn't a real answer, but it never hurst to ask.
Oh, it happens on any layered or laminate construction progressive lens, if you get my drift.

Rex "scratching my head, but not the lenses"

[offaxis]

a .25 cyl is just a wavy sphere anyway