Spectacle Skills

An assistant learns that a prescription is a set of numbers. A technician learns that those numbers describe a lens in a SPECIFIC position in front of the eye, and that moving the lens changes what the patient actually receives. Four position variables carry nearly every spectacle question at COT level: vertex distance, pantoscopic tilt, wrap (face-form) angle, and centration.

Vertex distance and effective power. The spectacle plane normally sits about 12 to 14 mm in front of the cornea. Move a lens along that axis and its effective power at the eye changes, according to Fe = F ÷ (1 − dF), where d is the change in distance in METERS and F is the lens power in diopters. Work an example: a −12.00 D spectacle lens at a 12 mm vertex becomes −12.00 ÷ (1 − [0.012 × −12.00]) = −12.00 ÷ 1.144 = −10.49 D, about −10.50, at the corneal plane. The direction is worth memorizing as a sentence rather than a formula: moving a lens CLOSER to the eye makes a minus lens act stronger and a plus lens act weaker. That is why a high myope's contact lens power is less minus than the spectacle Rx, and why an aphakic's contact lens is more plus.