Visual Acuity and Refraction Procedures
Acuity procedure
Standard distance acuity is measured at 20 ft (6 m). Occlude the fellow eye without pressing on the globe, and verify the patient is not peeking. Start above the expected threshold (20/40 or 20/50) and move to smaller lines. Record the smallest line on which the majority of letters are read correctly; the endpoint is not perfection. Test OD, OS, then OU, with and without correction. If acuity is worse than 20/200, move to 5 ft and adjust the recorded value, then count fingers, hand motion, light perception, no light perception. Use a pinhole to screen for uncorrected refractive error before concluding amblyopia or pathology.
Refraction sequence
Retinoscopy or autorefraction provides a starting point. In retinoscopy at 50 cm, subtract 2.00 D of working distance. Refine sphere to best acuity, then cylinder axis, then cylinder power with a ±0.25 Jackson cross-cylinder. The duochrome endpoint is equal or slightly red-preferring; over-minusing shifts preference to green. Perform binocular balance before finalizing.
Worked example
Retinoscopy at 50 cm shows +1.50 D sphere in both the 90 and 180 meridians. Subtract +2.00 D, giving −0.50 D sphere as the starting point for subjective refinement. The common error is forgetting the working-distance subtraction and prescribing +1.50.
Traps
Recording a line the patient did not actually read, or giving credit for guessing. Skipping the pinhole. Testing near acuity at the wrong distance (standard is 40 cm). Not documenting the method (projector, chart, near card) or the near add.