Abstract

Depth of focus of the human eye is determined by the loss of resolving power (visual acuity) with increase in out-of-focus blurring of the retinal image. An instrument is described which permits accurate measurement of these phenomena, using psychophysical methods with the checkerboard visual-acuity test chart. All test conditions are held constant, binocular vision is maintained, and time of exposure of the test target is limited to 0.2 second. Depth of focus was measured under the two following conditions: (1) increase in the angular size of the test target, and (2) increase and decrease of pupil size as effected by suitable drugs. With the angular size of the test target equivalent to 20/25 Snellen notation, for three subjects the mean total depth of focus was found to be 0.94 diopter at the 50% level of probable visual resolution of the target details and 0.63 diopter at the estimated 99% level. It was found that for each step of increase in target size (20/25 to 20/30, and so on) the total depth of focus increased by about 0.35 diopter. It was found also that over the range of pupil size from 2.5 to 8.0 mm the total depth of focus decreased about 0.12 diopter per millimeter of increase in the size of the pupil. These results were reviewed with respect to hyperfocal distance.

© 1959 Optical Society of America

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