Optics, Refraction and Contact Lenses
This domain tests geometric optics, refractive error, biometry, IOL power calculation, and contact lens fitting. Questions often combine a refraction with a clinical decision rather than asking for a formula alone.
Core optics
- Vergence: F = n/f; thin lens powers add. Myopia means the eye is too long or too powerful; hyperopia the reverse; astigmatism means unequal meridians.
- Spherical equivalent = sphere + ½ cylinder. With-the-rule astigmatism has the steeper vertical meridian, corrected by plus cylinder near 90°.
- Keratometry: D = 337.5 / radius in mm. Average K is about 43 D (7.8 mm).
- IOL power depends most on axial length; use optical biometry. Post-refractive surgery eyes need the history method or an ASCRS calculator, not standard formulas.
- Accommodation amplitude ≈ 18.5 − 0.3 × age (Hofstetter). Presbyopic adds follow this decline.
- Vertex distance matters above about ±4 D: a −10 D spectacle lens at 12 mm needs about −8.9 D at the cornea.
- Contact lenses: Dk/t governs oxygen transmission; silicone hydrogels have high Dk; RGP lenses are useful for irregular astigmatism and keratoconus; orthokeratology flattens the central cornea.
Worked example
A prescription is −4.00 −2.00 × 180. The spherical equivalent is −4.00 + (−1.00) = −5.00 D. The axis 180 indicates with-the-rule astigmatism because the vertical meridian is steeper. For a −10.00 D spectacle wearer switching to contact lenses, ignoring vertex distance would over-minus the contact lens and blur distance vision.
Traps
Spherical equivalent is not the cylinder power. Keratometry uses 337.5, not 375. Do not use standard IOL formulas after myopic LASIK. Axis 180 is not against-the-rule.