Prescription Analysis, Fabrication, and Verification
Reading and converting the prescription
Confirm OD and OS, sphere, cylinder, axis, add, prism and base, PD, and any base curve or thickness request. Transposition rule: new sphere equals old sphere plus old cylinder, new cylinder is the old cylinder with its sign reversed, and the new axis is the old axis plus or minus 90 degrees, kept between 1 and 180. Spherical equivalent is sphere plus half the cylinder.
Fabrication
Surfacing generates the back curve, then fining, polishing, coating, and edging to the frame shape. Prism can be produced by decentering the optical center or by surfacing prism into the lens. Slab-off or bicentric grinding compensates for vertical imbalance in anisometropia, and prism thinning reduces thickness differences in progressive lenses.
Verification
Lensometry confirms sphere, cylinder, axis, add power, prism, optical center location, PD, and thickness. ANSI Z80.1 sets tolerances: low-power sphere and cylinder tolerances are roughly a tenth of a diopter and loosen as a percentage at higher powers, while axis tolerance is looser for weak cylinders and tighter for strong ones. Prism must be within a small fraction of a prism diopter. Practical thickness minimums are about 1.0 mm at the edge for minus lenses and about 2.0 mm at the center for plus lenses.
Worked example
Convert +1.50 -2.50 by 060. New sphere is 1.50 minus 2.50, or -1.00. New cylinder is +2.50. New axis is 60 plus 90, or 150. Result: -1.00 +2.50 by 150.
Traps
Sign errors during transposition, changing the axis by an incorrect amount, verifying the add in the distance zone of a progressive, and assuming a frame is defective when the error is actually a measurement or PD error.