"As shown in Figure 7, a simple model A = k describes the normal relationship between acuity and defocus, where A is acuity, k is a proportionality constant, Bi is the intrinsic blur, and B is the external blur (defocus, in diopters) added to the test eye (Levi & Klein, 1990a, 1990b; Watt & Hess, 1987; Watt & Morgan, 1984)." . . . .