Once & for all: The bottom line on prism in wrap eyewear

[Barry Santini]

Makes yer choice:

Q: Wrap eyewear should not routinely have which of the following:

A. Base-in prism
B. Base out prism

If you'r answer is B, tell me why.

If your answer is "A", and you feel that "Current ANSI standards apply to wrap eyewear", go to the back of the class.

CURRENT ANSI PRISM STANDARDS SHOULD NOT APPLY TO WRAP EYEWEAR, PERIOD

Rant off!

Barry

[Leo Hadley Jr]

[Chris Ryser]

Horizontal Prism Tolerance Question [Archive] - OptiBoard ...

32 posts - Last post: 19 Sep 2007
"Flat" glasses typically don't demonstrate any prism error (or imbalance), even if plano. But a 22-26 degree pair of wrap eyewear (most ...
www.optiboard.com/forums/archive/index.../t-23900.html - Cached - Similar

[bob_f_aboc]

The successful fits that I have with wrap Rx eyewear all have between .25 and .75 Base In prism depending on the Rx.

The unsuccessful fits have been made with no prism readable in the lensometer.

[HarryChiling]

Barry, if your labs not gettin the job done get another lab. I find that eyewear needs to have prism compensation to work properly, often times on lower powers I will skip prescription compensation, but with prism it's a must.

[Barry Santini]

Barry, if your labs not gettin the job done get another lab. I find that eyewear needs to have prism compensation to work properly, often times on lower powers I will skip prescription compensation, but with prism it's a must.

I'm with you on this one, Harry. Wrap eyewear requires base in prism.

Period.

Barry

[HarryChiling]

Barry,

I'm guessing low powers and ANSI for prism being applied as mm not the actual prism induced. This can take prism from base in to base out and vice versa and still be in tolerance. Wrap eyewear isn't all that difficult once the few obstacles have been overcome. In the case with lower powers you may want to overcompensate the prism or reduce the amount of wrap in the eyewear if the lenses don't meet specs.

[drk]

Curious, Barry...

What has turned Dr. Banner into the Incredible Hulk on this issue? :bbg:



(I vote for 1/2 BI OU for wrap compensation, though I don't know why. Logic tells me BO.)

[Barry Santini]

drk:

It's cause many others insist on applying standard ANSI to wrap, but it makes no sense to me.

Example

Rx:

R -1.50 -0.25 80
L -1.25 sph

Compensated Rx done to make ann Oakley Gascan sunglass wrap:
The same Rx power, but 0.44BI O.U.
I, knowing how Oakley detests allwoing less the 0.75D prism values on the on-line order entry, decide to gain prism by decentration and order:

Rt PD: 3mm wider (2.5mm for prism, 0.5mm for to compensate for wrap-narrowing of frame PD)
LT PD: 4mm wider (3mm for prism; 0.5mm for as above)

This yields ordered PDs of 35 & 36. (stretching the PD on minus lenses, BTW, TIP:This is a *trick* to get wrap jobs passed some vendors narrow PD requirements for non-free-form SV lenses)

Job arrive from Joakley, and is found to have 0.25D BASE OUT O.U. at the requested, stretched PD.

There should *never* have been a tolerance here for BO; ONLY BASE IN.

Yes?

barry

[drk]

I feel your pain.

[Chris Bowers]

I’m confused. Doesn’t the image (following) reverse from base to apex when powers change between minus and plus?
How do we apply Barry’s rule to a Plano powered lens with non parallel surfaces?
As for ANSI, the people involved writing this are not slackers. Some reasons for the tolerances created are partially described in “Scope”. This describes real world possibilities and economic realities?
When reading the whole standard and judiciously apply this to a multi-billion dollar industry that has very little government oversight we see that it was not written for single issues but as a voluntary standard that tries propel an industry forward. For the nay sayer’s to follow, the lenses I make today far outweigh both in quality and complexity over the lenses I made 35 years ago. And in real term costs are the best value ever.
Do you really think you should send these fine people to the back of the class?