Horizontal Prism Tolerance Question

**Barry Santini** · 09-18-2007, 09:36 AM

Originally Posted by Darryl Meister

While wrap prism is often base in, the prismatic effect perceived by the wearer is identical to the prismatic effect produced by normal unwanted prism. There is no reason to apply the tolerance differently, unless you want to make the same argument for traditionally prescribed prism. Secondly, wrap prism technically is prescribed prism, because it is a prismatic component you are ordering that differs from the normal optical centration distance. Compensated prescription changes, often used for free-form progressive lenses, which are also based on tilt and wrap values, are also treated as prescribed changes in the ANSI standard.

I still somewhat disagree...

"Flat" glasses typically don't demonstrate any prism error (or imbalance), even if plano. But a 22-26 degree pair of wrap eyewear (most sports styles), even if plano, would manifest base out prism that would exceed our current ANSI tolerance. So an observer would see prism in this case...without being "prescribed". By allowing standards that would pass an error such as this is unaccetable, IMHO.

In fact, most makers of premium sports eyewear (such as Oakley), trumpet the fact that many competing models demonstrate "unwanted" prism...a result of the absence of a proper amount of *compensating* base in being present. This results is a base-out error.

In the end, I feel it is our current refractive paradigm that is outdated, not ANSI...

Barry

**HarryChiling** · 09-18-2007, 11:05 AM

Originally Posted by Darryl Meister

1. The ANSI power tolerances have remain virtually unchanged since 1972 for spherical lens designs (well, the power tolerances went from 0.12 D to 0.13 D). Otherwise, the only significant changes since the 1979 standard were a change in the cylinder axis tolerances to be more consistent with the power tolerances and the addition of separate tolerances for progressive lenses.

2. Prior to 1972, neither plastics nor tempered glass lenses were in use. Moreover, the power tolerances in the original 1964 standard (0.06 D) actually pushed the envelope of what a traditional surfacing process could theoretically deliver in the first place, even if all other factors were spot on, since the precision of a surfacing process utilizing eighth-diopter tooling is only 0.0625 D. Not to mention the argument against the rationale of having a fabrication tolerance that was 4 times tighter than the just-noticeable-difference of the observer and smallest increment typically used during the actual refraction (0.25 D).

Good points, I guess I don't exactly know all the variables considered when the ANSI's were drafted, but I do on occasion see refractions that were performed in 0.12D and you ar correct that nowadays 99% of the scripts I see are performed in 0.25D steps, but if the refractionist for instance rounded the measure to 0.25D steps when the patient may have benefited from a 0.12D more or less power than wouldn't the ANSI's additional 0.12D compound that error?

I also remember that the tolerances have a percentage of patients that cannot adapt to this error and although the number is small would it go up if we consider the refraction into the equation?

Interesting thread thanks all.

**Darryl Meister** · 09-18-2007, 05:14 PM

"Flat" glasses typically don't demonstrate any prism error (or imbalance), even if plano. But a 22-26 degree pair of wrap eyewear (most sports styles), even if plano, would manifest base out prism that would exceed our current ANSI tolerance. So an observer would see prism in this case...without being "prescribed". By allowing standards that would pass an error such as this is unaccetable, IMHO.

I'm not saying that the unwanted prism induced by lens tilt is considered prescribed prism, I'm saying that any prism you order to neutralize this unwanted prism is considered "prescribed" prism. The Z80.1 standard is a fabrication standard, not a design standard (although the subcommittee tried that in the first two standards). This induced prism is no different than any other optical quantity affected by the position of wear, such as vertex distance, pantoscopic tilt, reading distance, or optical center height relative to the primary gaze.

For instance, that same lens and frame combination may be sitting 5 mm away from the refracted vertex distance, or introduce significant unwanted cylinder power errors as a result of len tilt. In either case, significant power changes may occur if the eye care professional has not taken steps to modify the prescription accordingly, but these do not represent fabrication errors. If the lenses were made according to a standard prescription, and you were then to measure these lenses in a focimeter, which is the test method used in spectacle lens standards, they would look exactly as ordered.

**Darryl Meister** · 09-19-2007, 12:35 AM

but if the refractionist for instance rounded the measure to 0.25D steps when the patient may have benefited from a 0.12D more or less power than wouldn't the ANSI's additional 0.12D compound that error?

Yes, but unfortunately this is just the nature of manufacturing. While "no tolerance" is a nice concept in theory, it is not an especially realistic one. Besides, as I noted before, the just noticeable difference -- or depth of focus -- for a typical observer under normal viewing conditions is around 0.25 D, which is why this value generally represents the smallest measurement used in refraction. Even if, in your example, the patient would have been exactly in the middle of two 0.25-diopter steps, that still means that the maximum error with a 0.13 D fabrication tolerance is still no more than 0.25 D.

That said, I'm all for quality and accuracy. But not necessarily everyone in this industry feels that lenses should adhere to exceptionally tight tolerances, particularly if it will drive the cost of eyewear up significantly while offering only minimal visual benefit to the wearer.

**Barry Santini** · 09-19-2007, 08:24 AM

Originally Posted by Darryl Meister

That said, I'm all for quality and accuracy. But not necessarily everyone in this industry feels that lenses should adhere to exceptionally tight tolerances, particularly if it will drive the cost of eyewear up significantly while offering only minimal visual benefit to the wearer.

I'd have to whole-heartedly agree with you on this point.

That's why I advocate revisting the current paradigm in refraction...

...along with a better understanding and acceptance of the factors in neural adaptation

Barry

**Barry Santini** · 09-19-2007, 08:53 AM

Originally Posted by Darryl Meister

If the lenses were made according to a standard prescription, and you were then to measure these lenses in a focimeter, which is the test method used in spectacle lens standards, they would look exactly as ordered.

First of all, I want to thank you, Darryl, for taking the time to offer a larger perspective on this subject than those of us *infantry* normally encounter "on the front line."

For my money, in the end its is not whether eyewear "looks exactly as ordered", but rather, performs exactly as desired.

It seems more education and training for refractionists is in order. (The following thought may be suitable for a new thread)

Why is it that optometrists must have a 4 year undergraduate degree in (preferably) pre-med, and spend alot of their 4 years of optometry school on the vision system (including refraction)....

While Ophthalmologists (and related techs) are exposed to far less training in refraction and related concepts...

yet both disciplines are expected by the public (and dispensers) to have a mastery in this aspect of eye exams that the public thinks *is* the eye exam.

Thoughts?

Barry