prism checking point on varifocals

[yzf-r1]

Why is the prism-checking point on varifocals level with the nasal and temporal reference circles and not on the fitting cross?

[Darryl Meister]

There are probably several reasons for this, though I imagine that a lot of it is based upon the premise of "Because that's the way we've always done it," anymore. However, there are some definite advantages to this approach:

1. Greater blank size cut-out
2. Less vertical imbalance at near for anisometropic prescriptions
3. Better cosmetics in many lenses without prism-thinning
4. Reduced chance of blocking waves affecting the near zone
5. Closer match to the vision provided by single vision and bifocal lenses, since the OCs are generally at the datum line -- several millimeters below the pupil
6. At one time, there might have been a practical advantage (in terms of calculations or computer processing) to keeping the critical regions of the lens close to the center of the design, though I would only be speculating on this one

Best regards,
Darryl

[Joann Raytar]

I have a question about the PRP. Even if there is prism thinning, shouldn't ground prism still fall within ANSI tolerance at the PRP?

[shanbaum]

The Z80.1 standard includes a special condition for prism thinning: In the cases where prismatic thinning is used, the prism thinning prism is considered to be a prescribed prism.

So, the answer is, yes and no.

[Joann Raytar]

Hmm. I received a pair of lenses, patient is a hyperope +2.25/+2.50, with 3D base down OU. I thought it was a bit too much prism, the lab said it was correct and just the result of prism thinning. I haven't seen a thinned lens at 3D Dn unless they were blocked wrong.

So you are saying the lab could be right?

[CME4SPECS]

Jo, you don't mention anything other than power. But, if the seg height is way above the 180 line more prism is neccessary to accomplish equal thickness at top and bottom. Terry Agin

[Joann Raytar]

Ahhh, I don't remember the actual eyesize but it was a large sized, double bridge men's frame. I want to say the fitting height was something like 28-30, can't remember exactly.

[Darryl Meister]

Most of the more sophisticated lab processing programs have a limit you can set on prism-thinning, which is often set at 3.0 prism diopters or less. According to a study done a few years back, wearers can tolerate 2.0 prism diopters of vertically yoked prism (which is essentially what prism-thinning is), but generally reject 4.0 prism diopters. So, 3.0 sits right in the middle. However, from a practical stand-point, you should also keep in mind that these values are based upon the prism reference point location, and additional prism may be incurred at the fitting cross.

Best regards,
Darryl

[shanbaum]

Jo said:

So you are saying the lab could be right?

Sure. If you want prism thinning to be limited to something less than your lab normally applies, let them know (or, just "prescribe" the amount you want). There really isn't a standard for the amount of prism thinning.

I think that the typical cap that labs put on thinning is 3.0D.

[CME4SPECS]

My former boss's wife was a +5 or so, she always wore her frames very low and the frames were very deep, so the segs were always 8 or 9 above. She always complained of the thickness at the top, though they had BD prism. To equalize the thickness I often put 5-6 prism diopters to balance the thickness, she did fine with it.
I use to make glasses for an engineer that would put on his PALS and sit and watch the the table top jump up a half inch, he would get all wound up over it and would not accept any equalizing prism in his glasses. So one never knows who can tolerate what!
Terry Agin