Optics, refraction, and contact lenses
Optics and refraction questions test whether you can move fluently between vergence, lens power and the patient's symptom, and whether you know when a contact lens is a clinical decision rather than a cosmetic one. The contact lens part concentrates on oxygen transmissibility, fit and hygiene-related disease.
Vergence and the reduced eye
Light from infinity has zero vergence; a surface adds its power, and moving a distance d divides vergence. The reduced eye is a single 60 dioptre surface with axial length 22.2 mm; 1 mm of axial error is worth about 3 dioptres. Myopia means the eye is too long or too powerful, hyperopia the reverse. Spherical equivalent equals sphere plus half the cylinder, so minus 3.00 with minus 1.50 at 180 becomes minus 3.75. With-the-rule astigmatism has the steeper meridian at 90 degrees, against-the-rule at 180.
Clinical optics you must calculate
Vertex distance matters above about 4 dioptres: a minus 10 dioptre spectacle lens moved closer to the eye becomes effectively stronger for a myope. Accommodation amplitude declines with age and presbyopia appears around 40 to 45 years. Retinoscopy finds the far point; keratometry gives corneal power; IOL power comes from axial length, keratometry and anterior chamber depth, with modern formulas such as Barrett outperforming SRK/T in long and short eyes.
Contact lenses
Oxygen performance is Dk/t, transmissibility, not Dk alone; silicone hydrogels meet daily wear needs easily and extended wear needs more. Soft lenses drape; rigid gas-permeable lenses need apical clearance and mid-peripheral bearing on a cone in keratoconus. Orthokeratology flattens the central cornea overnight and is reversible. Complications include microbial keratitis (Pseudomonas with soft lenses, Acanthamoeba with water exposure), giant papillary conjunctivitis and hypoxia-related neovascularisation.
Worked example
A minus 9.00 dioptre myope wearing spectacles wants contact lenses. Reasoning: at the corneal plane the required power is lower in magnitude, roughly minus 8.00, because the vertex distance is removed; you must not simply transfer the spectacle power. Then check keratometry and tear film to choose a soft toric or a rigid lens.
Traps
Quoting Dk when asked for transmissibility; ignoring vertex distance in high myopia; treating astigmatism as a single number rather than a meridian-specific error.