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A prescription of −1.00 +0.25 × 180 describes a lens that has a horizontal power of −1.00 D and a vertical power of −0.75 D. Only ophthalmologists write prescriptions in + cylinder. An optometrist would write a prescription in - (minus) cylinder. All spectacle and contact lenses would be made in minus cylinder.
Example 1: example prescription adjustment from glasses to contacts [ edit ] A phoropter measurement of a patient reads −8.00 D sphere and −5.25 D cylinder with an axis of 85° for one eye (the notation for which is typically written as −8 −5.25×85 ).
Lensmeters can also verify the power of contact lenses, if a special lens support is used. The parameters appraised by a lensmeter are the values specified by an ophthalmologist or optometrist on the patient's prescription: sphere, cylinder, axis, add, and in some cases, prism.
A torus results when a circle with radius r rotates around an axis lying in the same plane as the circle (here the z axis) at a distance R from the centre of the circle.. A torus is the surface of revolution resulting when a circle with radius r rotates around an axis lying within the same plane as the circle, at a distance R from the circle's centre (see figure at right).
The values indicated in the sphere and cylinder columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A diopter is the reciprocal of the focal length in meters.
While in principle aspheric surfaces can take a wide variety of forms, aspheric lenses are often designed with surfaces of the form = (+ (+)) + + +, [3]where the optic axis is presumed to lie in the z direction, and () is the sag—the z-component of the displacement of the surface from the vertex, at distance from the axis.