by Daniel Torgersen OLA Technical Director
ANSI Z80.1-2005, American National Standard for Ophthalmics – Recommendations for Prescription Ophthalmic Lenses, was given final approval by ANSI with an effective date of December 19, 2005. This revision replaces ANSI Z80.1-1999 and contains two significant changes: the tolerance on cylinder axis and the toleranceon progressive lens refractive power.
Tolerance on Cylinder Axis
A comparison of ANSI Z80.1-1999 and ANSI Z80.1-2005 shows the following tolerances on cylinder axis.
Tolerance on Direction of Cylinder Axis
Nominal Value of Cylinder (D) Axis Tolerance ANSI Z80.1-1999 Axis Tolerance ANSI Z80.1-2005
< 0.25 ± 7° ± 14°
.375 ± 7° ± 7°
.50 ± 5° ± 7°
> 0.50 to < 0.75 ± 5° ± 5°
>0.75 to < 1.50 ± 3° ± 3°
> 1.50 ± 2° ± 2°
Basis for the Change
It is apparent that the tolerance on the cylinder axis for cylinders below 0.75 D has been increased. The Z80 Committee approached the tolerance on cylinder axis by asking how far the axis must be shifted in order to introduce an error in the cylinder power equivalent to a cylinder power tolerance of 0.12 D. This can becalculated by the following equation. α = sin-1 (C / 2F1) Where: α = the angle C = resultant cylinder power error (0.12 D) F1 = prescribed cylinder power
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OLA White Paper ANSI Z80.1-2005
The graph below illustrates the result of this calculation. This result was used to establish the tolerance for the low cylinders (0.50 D and less).
Axis Error Required to Induce a 0.12 DCylinder Power Error
Cylinder Axis Error (D)
Prescribed Cylinder Power (D)
There is precedence in using this approach. Dr. Glen Fry used a similar methodology to arrive at the same conclusions in 1977 and was published in Optometric Weekly in an article entitled “Tolerancesfor Cylinder Axis”. That analysis expanded the tolerances in the 1972 standard to those of the 1979 standard. ANSI Z80.1-1999 used the same tolerances as the 1979 standard. Despite the fact that Dr. Fry’s analysis indicated that the cylinder axis tolerance for low cylinder powers should be expanded to those indicated in the above graph, the ANSI Z80 Committee at that time thought the expansionwould be too extreme to be accepted.
Why Change Now?
In 2004 the majority of presbyopes used progressive lenses. It is common that progressive lenses have small amounts of unwanted cylinder at the distance reference point. This is recognized in ISO standards for semi-finished blanks which allow more cylinder for progressive lenses than standard multifocal and single vision semi-finished blanks.The
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OLA White Paper ANSI Z80.1-2005
presence of even a small amount of cylinder can significantly change the prescribed cylinder axis. Consider an Rx which has prescribed cylinder of -0.25 D @ 180 0. This Rx is ground into a semi-finished blank having unwanted cylinder of -0.09 D @ 45 0 ( which is equal to the ISO tolerance). The result, as shown below in Figure 1, isa crossed cylinder with a resultant cylinder of -0.26 D @ 10 0.
Figure 1 – Resultant Cylinder and Cylinder Axis
Effect on the Wearer
How might the increased tolerance on cylinder axis affect the wearer? As noted above, a 140 axis error for a 0.25 cylinder produces a cylinder power error of 0.12 D. The 1999 standard allowed a 70 axis error which produces a cylinder power error of 0.06 D.Consequently, the difference in cylinder power error between the two standards is 0.06 D. How does the cylinder power error affect the wearer? Simply stated, it moves the circle of least confusion by one-half the amount of the error as shown in Figure 2. The 1999 standard allowed a 70 axis error for a -0.25 D cylinder which produces a cylinder power error of 0.06 D. Consequently, the wearer will...