Snellen acuity is determined as a functional of refractive error for two subjects under cyploplegia, using randomised test charts, constant test chart luminance and in serise of artificial pupils. These results allow determination of the depth-of-focus of the eye for Snellen targets, and it is shown that both optical and retinal factors influence the depth of-focus achieved. Depth-of-focus is found to increase with decreasing pupil diameter and visual acuity. The clinical significance of these results, particularly with respect to the precision with which refractive error may be determined, is discussed.