Functional Vision Evaluation: Measuring What the Person Actually Uses
What the evaluation is for
A functional vision evaluation asks not only how much vision is present but how the person uses it in real tasks. Candidates must know the standard measures and their limits: distance acuity (Snellen, ETDRS, Feinbloom for low ranges), near acuity (M units, Jaeger), contrast sensitivity (Pelli-Robson letter chart, Mars letter chart, grating tests), visual fields (confrontation, tangent screen, Amsler grid for central distortion, perimetry for peripheral loss), color vision (Ishihara for red-green; HRR or D-15 for acquired loss), and stereopsis (Randot, Titmus, Frisby). Visual perceptual and visual motor skills (visual closure, figure-ground, spatial relations, eye-hand coordination) and literacy-reading rate, magnification needs, handwriting, and compensatory listening or braille-complete the picture.
Traps to watch
- Confusing acuity with function: a person with 20/200 may read with magnification while someone with 20/40 and dense field loss cannot navigate. Always ask which task the finding predicts.
- Mixing measurement scales: Jaeger is printer-dependent; M units are more defensible for near. Feinbloom numbers are not Snellen equivalents.
- Forgetting illumination and glare change results; record lighting conditions.
- Confusing Ishihara (congenital red-green screening) with tests appropriate for acquired color loss.
- Reporting a field defect without describing its functional consequence (e.g., inferior field loss and stair falls).
Rule of thumb
Match each test to the task it predicts, and always translate a number into a functional statement ("reads 1M print at 40 cm with +4D add under 500 lux").
Worked example: A client reads 1M print at 25 cm. Required acuity for 1M at 40 cm is roughly 1/40 = 0.025 (about 20/800 equivalent demand). Because they read at 25 cm, the demand is 1/25 = 0.04. The gap between demand and their acuity guides the magnification needed-not a memorized formula alone.