The Step Is the Accuracy: How Trial-Lens Step Size Caps Self-Selected Correction, and a Two-Stage Kit That Beats It

Where no refractionist is available, wearers can be offered trial lenses and asked which is clearer - but the fineness of the set decides in advance how good the result can be. This paper computes the ceiling that the step size imposes. A set in 1.00 D steps leaves a worst-case residual error of 0.5 D, costing about 0.2 logMAR of acuity by a stated clinical rule of thumb; a 0.50 D set halves that to 0.25 D and 0.1 logMAR, at 25 lenses per eye form over a plus-or-minus 6 D range; a 0.25 D set reaches dispensing-grade worst case 0.125 D but doubles the swapping again to about 8.0 minutes of comparisons, past the point where a wearer's judgements stay reliable. The specification is a two-stage set: coarse 1.00 D lenses to bracket, then 0.25 D lenses only around the bracket, which buys quarter-step accuracy for the price of the half-step set. This study is simulated, not measured.