Tests for visual acuity in general measure the macular function of the light-adapted eye. Though the binocular field of vision extends to 184 degrees, the true macular field does not exceed 2 degrees (Fergus1). The factors determining visual acuity in a healthy eye are chiefly refractive error, visual angle, illumination, or rather brightness, contrast and period of exposure. Subordinate factors are irradiation, color, glare, width of pupil, attention and fatigue.
FACTORS IN VISUAL ACUITY AND ITS DETERMINATION
Illumination.—Illumination in a definite manner affects visual acuity. A person who observed a chart illuminated by 1 foot-candle disclosed a visual acuity of 0.98 minute; when the illumination was increased to 100 foot-candle, the visual acuity improved to 0.61 minute (Luckiesh and Moss2).
At the lowest intensities of illumination, the perimacular field is more sensitive than the fovea, and fixation is peripheral. If one constructs a curve
LEBENSOHN JE. FACTORS IN THE DETERMINATION AND INTERPRETATION OF VISUAL ACUITY. Arch Ophthalmol. 1933;10(1):103–109. doi:10.1001/archopht.1933.00830020111019
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