Refraction, Retinoscopy and Prescription of Optical Aids
Concepts
Master the types of ametropia (myopia, hyperopia, astigmatism, presbyopia, anisometropia), the far point and near point, and the distinction between spherical equivalent and the actual sphero-cylindrical prescription. Understand Jackson cross-cylinder refinement, the duochrome test, and the fogging technique. For retinoscopy, know the working distance effect (subtract working distance in dioptres: at 50 cm subtract 2.00 D; at 67 cm subtract 1.50 D), with-motion vs against-motion, and the use of cycloplegia in children.
Traps
Examiners love the sign convention: with-motion in retinoscopy means the patient is hyperopic or under-corrected myopic relative to the working distance. Another trap is confusing the cylinder axis with the power meridian (axis is 90° from the correcting meridian). Also watch for transposition errors: +2.00/−1.00×180 transposes to +1.00/+1.00×090.
Worked example
Retinoscopy at 50 cm shows neutrality with a +3.00 D lens in the vertical meridian and +1.00 D in the horizontal meridian. Subtract the working distance (+2.00 D) from each: vertical = +1.00 D, horizontal = −1.00 D. The prescription is +1.00/−2.00×180 (or transposed +1.00/+2.00×090).