Reading acuity—an important parameter of reading performance

Abstract

Visual acuity is the most widely used visual parameter. Reading is a widely expressed goal of patients with vision loss. Yet, reading acuity is often not properly understood and not properly calculated. Reading distances are often estimated and the letter size indicators used do not relate to those used on letter charts. Visual acuity is the reciprocal of the need for angular magnification. Subjects who need letters that are twice as close or twice as large are said to have a visual acuity of 1/2 (0.5, 6/12, 20/40); visual acuity for those that need 5× magnification is said to be 1/5 (0.2, 6/30, 20/100), etc. Snellen's formula: V=distance/letter size is awkward for reading vision, since the numerator becomes a fraction-within-a-fraction. When the formula is inverted to calculate 1/V (the magnification need), and when 1/distance is replaced by “distance in diopters,” which indicates the required reading add, the result is easier to use. The Modified Snellen Formula becomes: 1/V=letter size (in M-units)×distance (in diopters). This modified formula reveals relationships that are otherwise hidden. This includes the difference between functional reading acuity (magnification required for prolonged reading) and threshold reading acuity.