TOLERANCES ON THE POWER OF LENSES*
The tolerances apply to the back vertex power and should be measured at the optical centre of a single vision lens, distance centre of a bifocal or the distance checking circle of a progressive lens. The tolerance of the power for each meridian is determined by the highest absolute meridian power. Use column A to check that the meridians are within tolerance (e.g. single vision lens power S -4.75 C -2.25, highest meridian is -7.00. The tolerance on -7.00 is ± 0.12; tolerance on -4.75 is ± 0.12).
To check that cylinder power is within tolerance, use column B (e.g. single vision lens power S -4.75 C -2.25, highest meridian is -7.00, cylinder is -2.25 and tolerance is ± 0.18).
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Accuracy of optical centration and prismatic power should be assessed at the nominal geometric centre of a single vision lens, the distance optical centre of a bifocal lens or the prism reference point of a progressive lens.
NOTE: Progressive lenses from Essilor incorporate a base down cosmetic thinning prism equal to two- thirds of the reading addition. This should be taken into account when checking any prescribed prism.
Prescriptions compensated for the "as worn" position
Certain Essilor lens designs have the lens prescription modified for the "as worn" position when fitted to the frame or mount. These include all Varilux Ipseo New Edition, Varilux Physio 2.0 f-360° and Varilux Physio 2.0, Varilux Comfort New Edition and all lenses with eyecode.
With such lenses the prescription when measured on a focimeter differs from the ordered prescription.
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Lenses with compensated prescriptions are supplied in lens packets displaying the ordered prescription first and the compensated prescription below.
Ordered
Sph +3.75 Cyl +0.75
Axis 30 Add 2.75
Compensated
Sph +3.71 Cyl -0.61 Axis 39
The prescription measured at the power checking circle is a combination of the compensation and the prescribed power. The wearer will perceive the prescription as the prescriber intended it. It is recommended that the lens packets should be forwarded to the final point of checking in the practice so the measured values are visible for this purpose. When the measured cylinder value is 0.20D or less, the axis becomes impractical to measure and so is not displayed.
* ISO 8980-1 & 2 |
