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The axis defines the location of the sphere and cylinder powers. The name "axis" comes from the concept of generating a cylinder by rotating a line around an axis. The curve of that cylinder is 90° from that axis of rotation.
From the measurements taken, the specialist will write an eyeglass prescription that contains at least three numerical specifications for each eye: sphere, cylinder, and axis, as well as pupillary distance (distance between eyes), and, rarely, prism for one or both eyes.
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. The lensmeter is also used to check the accuracy of progressive lenses , and is often capable of marking the lens center and various other measurements ...
The axis value does not change with vertex distance, so the equivalent prescription for a contact lens (vertex distance, 0 mm) is −7.30 D of sphere, −4.13 D of cylinder with 85° of axis (−7.30 −4.13×85 or about −7.25 −4.25×85).
By measuring this zone, the autorefractor can determine when a patient's eye properly focuses an image. The instrument changes its magnification until the image comes into focus. The process is repeated in at least three meridians of the eye and the autorefractor calculates the refraction of the eye, sphere, cylinder and axis.
Refractive errors are typically measured using three numbers: sphere, cylinder, and axis. [19] Sphere: This number denotes the strength of the lens needed to correct your vision. A "–" indicates nearsightedness while a "+" indicates farsightedness. Higher numbers indicate more power in either direction. Cylinder: This number denotes the ...