Ophthalmic Optics and Lensometry
Lensometry
Neutralize the distance portion first: sphere, then cylinder axis, then cylinder power; then the near add. Confirm the optical center and measure binocular and monocular pupillary distances. Check prism by target displacement and verify with Prentice's rule: prism in diopters equals decentration in centimeters times power in diopters. Vertex distance matters beyond plus or minus 4.00 D, since a refraction at 12 mm converted to a contact lens or a different frame changes the effective power. Transpose between plus and minus cylinder as needed.
Optics concepts
Sagitta: s equals r minus the square root of (r squared minus half the diameter squared). Higher-index materials allow thinner lenses but lower the Abbe value. Polycarbonate is impact resistant and is required for children and monocular patients. Progressive lenses have a fitting cross and a corridor; segment height is measured relative to the lower lid.
Worked example
A +8.00 D lens is decentered 2 mm per eye from the optical center in a 64 mm pupillary distance frame. Prentice: 0.2 cm times 8.00 equals 1.6 prism diopters base-out per eye, 3.2 total, enough to cause asthenopia. Recenter the lenses or prescribe compensating prism.
Traps
Not calibrating the lensometer. Reading the axis 90 degrees off. Recording power before neutralizing prism. Ignoring vertex distance for high prescriptions. Not verifying the near add or segment height.