Optics, Refraction and Ophthalmic Dispensing
What this domain tests
Optics and dispensing questions test your ability to calculate lens power, predict the effect of vertex distance and prism, and translate a prescription into a comfortable, correctly centred spectacle.
Core optics
- Thin-lens power F = 1/f (metres); two thin lenses separated by distance d combine as F = F1 + F2 - dF1F2.
- Snell's law: n1 sin θ1 = n2 sin θ2; refractive index differences drive refraction.
- Prentice's rule: prism Δ = c (cm) × F (D), so a +6.00 D lens decentred 4 mm induces 2.4Δ.
Vertex distance and dispensing
- Effective power at the eye: F_eye = F/(1 - dF), with d in metres.
- Reducing vertex distance weakens the effective power of both plus and minus lenses; high prescriptions must be recalculated.
- Align the optical centre with the visual axis; account for pantoscopic tilt, face form and bifocal segment height.
- Transpose toric prescriptions correctly: sphere + cylinder, axis ±90°, cylinder sign changed.
Worked example
A patient wears -12.00 DS at a 12 mm vertex distance and is refitted at 10 mm. Original effect at the eye = -12/(1 - 0.012 × -12) = -10.49 D. For the new position, F = -10.49/(1 - 0.010 × -10.49) ≈ -11.75 D. Prescribe about -11.75 DS and verify with over-refraction.
Traps
Forgetting vertex compensation in high myopia; transposing the axis incorrectly; decentring a high plus lens and creating unwanted prism; ignoring frame tilt when checking centration.