SUBJECTIVE DEPTH-OF-FOCUS OF THE EYE

An experiment is described in which the subjective depth-of-focus (DOF ) of the eye, defined as the range of focusing errors for which the im age of the target appears to have the same clarity, contrast, and form as the optimal in-focus image, was measured as a function of the size of high contrast (99%) Snellen Es for 5 trained subjects under cyclop legia. Mean DOF increased by approximately 60% as the size of the lett er detail increased from -0.2 to 0.87 log min are (Snellen equivalent: 6/3.8 to 6/45), although there were considerable intersubject variati ons. DOF declined with increasing pupil diameter, the mean total DOFs being 0.86, 0.59, and 0.55 D for 2-, 4-, and 6-mm pupils, respectively , In a second experiment, use of low (21%) contrast letters with a 4-m m pupil and 4 subjects marginally increased the DOF (by 0.08 +/- 0.05 D); refraction also shifted in a myopic direction by a mean of 0.15 +/ - 0.06 D compared with the high contrast letters. A third experiment w ith four less-experienced subjects demonstrated the importance of inst ruction and training in any measurement involving judgment of just-per ceptible defocus blur. The clinical implications of the results for me asurements of refraction and amplitude of accommodation are discussed.