We assessed the influence of dioptric and diffusive blur on normal subjects' letter and Vernier acuity in two experiments. In the first experiment, letter acuity was measured for isolated black Sloan letters and Vernier acuity was determined for a pair of black vertical abutting or nonabutting lines. Targets were viewed through plus lenses that produced 0, 1, 2, 4, and 8 D of dioptric blur. In the second experiment, letter acuity was determined for bright 4-position Ts and Vernier acuity was measured for a pair of bright abutting vertical lines. Six levels of imposed diffusive blur were produced by varying the distance between a ground glass screen and the oscilloscope on which the targets were presented. The results of both experiments indicate that letter and Vernier acuity for abutting or closely separated lines worsen in parallel curvilinear fashion, as long as the lines comprising the Vernier targets remain equally detectable when various amounts of dioptric and diffusive blur are imposed. We conclude that both dioptric and diffusive blur introduce common processing limitations for letter and Vernier acuity.