Optics, Refraction and Clinical Optics
Core concepts
Master vergence, the thin-lens equation (1/f = 1/v − 1/u), and the relationship between dioptres and focal length. Understand emmetropia, myopia, hyperopia, astigmatism, and presbyopia. Know that the eye's total power is ~60 D, with cornea ~43 D and lens ~20 D; accommodation increases lens power.
Clinical optics: Snellen acuity, pinhole testing (improves refractive error, not media/retinal disease), and the duochrome test. Understand the difference between spherical equivalent and astigmatic correction, and how contact lenses differ from spectacles in vertex distance. Know the principles of retinoscopy (neutralisation) and autorefraction.
Traps
- Confusing myopia (light focuses in front of retina) with hyperopia (behind).
- Forgetting vertex distance changes with high-power lenses.
- Assuming pinhole improves all blurred vision (it does not improve media opacity or retinal disease).
- Miscalculating spherical equivalent (sphere + half cylinder).
Rule of thumb
Spherical equivalent = sphere + ½ cylinder. Use it to estimate the power needed when a patient cannot tolerate full astigmatic correction.
Worked example
A refraction of −2.00/−1.00 × 180 has a spherical equivalent of −2.50 D. If the patient is intolerant of cylinder, start with −2.50 D sphere.