Optics, Refraction and Contact Lenses
Optics, refraction and contact lenses is where arithmetic meets the trial frame: the examiner expects you to convert a retinoscopy finding or a spectacle prescription into a working contact lens power, and to explain why a patient who sees 6/6 in glasses may not tolerate the same power on the eye.
Core rules
- Reduced vergence L = n/l, with l in metres. Refractive indices to remember: air 1.000, cornea 1.376, aqueous and vitreous 1.336.
- Keratometry: K in dioptres = 337.5/r(mm). This uses the keratometric index 1.3375 and is not the true anterior surface power, which would use 376/r.
- Effective power: F_eff = F/(1 - dF), with d in metres. A plus lens brought closer to the eye becomes more plus; a minus lens becomes less minus. Running the equation backwards converts a spectacle power to a contact lens power.
- Toric transposition: sphere plus cylinder at an axis; spherical equivalent = sphere + half the cylinder. With-the-rule astigmatism has the steeper meridian at 90 degrees.
- Retinoscopy neutrality is reached at the working distance; subtract that distance in dioptres (67 cm = 1.50 D). Neutralise the two principal meridians separately.
- Soft lens fit is judged by centration, movement and coverage. A steep rigid lens shows central bearing with peripheral pooling; a flat lens decentres and traps bubbles.
Worked example
A spectacle prescription is +10.00 D at a vertex distance of 12 mm. What contact lens power is needed? F_eff at the cornea = 10/(1 - 0.012 x 10) = 10/0.88 = 11.36 D, so the nearest lens is +11.25 D. The reasoning is that the plus lens now sits on the eye, so its focal point must be brought forward by shortening the distance, which requires a stronger lens.
Traps
Reversing the vertex conversion when a lens is moved away from the eye, forgetting that a toric soft lens needs a stable axis, and calling a tight lens loose because it has little movement while the patient reports blurred vision on waking.