Ophthalmic Optics and Refraction
Core framework
Build from vergence: light from infinity has zero vergence; a lens changes vergence by its power. Know thin-lens equation, the relationship between focal length and power (P = 1/f in meters), and how to convert between plus and minus cylinder.
Concepts to know cold
- Refractive errors: myopia (minus), hyperopia (plus), astigmatism (cylinder), presbyopia (loss of accommodation).
- Spherical equivalent: sphere + half the cylinder. Useful for estimating needed power.
- Transposition: change cylinder sign, add it to sphere, rotate axis by 90 degrees. Example: +2.00 −1.00 × 180 becomes +1.00 +1.00 × 090.
- Vertex distance: matters for powers above about ±4.00 D; moving a lens closer to the eye changes effective power.
- Prism: Prentice's rule, P = cF, where c is decentration in cm and F is lens power in diopters.
Traps
Axis rotation is the most common error: always add or subtract 90 and keep the axis between 0 and 180. Watch units in Prentice's rule (centimeters, not millimeters). Remember that plus lenses converge and minus lenses diverge.
Rule of thumb
Transpose in three steps: flip the cylinder sign, add the old cylinder to the sphere, rotate the axis 90 degrees. Recheck by transposing back.
Worked example
A lens is decentered 4 mm with power +6.00 D. P = cF = 0.4 cm × 6.00 = 2.4 prism diopters, base toward the decentration direction. This explains induced prism complaints when optical centers are off.