Contact Lenses

A contact lens sits on the eye, not 12 mm in front of it, and it floats on a film of tears that is itself a lens. Almost every arithmetic error a technician makes in this area traces back to forgetting one of those two facts.

Vertex compensation. A lens moved closer to the eye changes its effective power. The exact relationship is Fc = Fs ÷ (1 − d × Fs), where Fs is the spectacle power, d is the vertex distance in meters, and Fc is the power at the corneal plane. The direction is the part worth burning in: a minus lens moved closer becomes more effective, so less minus is ordered; a plus lens moved closer becomes less effective, so more plus is ordered. Convention applies the compensation at roughly 4.00 D and above, because below that the correction is smaller than the smallest step normally dispensed. In a sphero-cylindrical prescription each principal meridian is compensated separately, which is why the cylinder on the lens order rarely matches the spectacle cylinder exactly: in a minus prescription the compensated cylinder comes out smaller (−8.00 −2.00 at 12 mm becomes about −7.25 −1.62), while in a plus prescription it comes out larger (+8.00 +2.00 becomes about +8.75 +2.50).