Stereoacuity has been one of the most popular fields in vision research, and is routinely evaluated in clinical practice and imaging technology. However, stereoacuity must have some necessary conditions, and the evaluation metrics must be carefully designed and executed, since stereopsis varies considerably between individuals, and test plates differ in terms of measurement conditions. Based on our preliminary study and past reports, this article reviews the necessary conditions and permissible values for maintaining stereoacuity.

The purpose of this study is to clarify the process of distinct imaging by normal eyes. The device used in the experiment was the improved Harvard Type Tachistoscope, re-modelled by the author to enable free setting of distances. Time till distinct imaging was measured at 5m under 350 lux; Landolt targets were exposed in 100 msec steps (by limiting methods) against each other, from 0.1 to 1.0 (decimal notation) subjects comprised 75 students, 17～21 years of age (naked eyes, 5m distance, all subjects 1.0+ in right and left visual acuity test of school). Eyes below 1.0 visual acuity and the eyes showing exceptional (abnormal) times in the distinct image experiment were excluded from analysis. The following results were obtained:

(1) Required time for distinct imaging was 200～300 msec, including all blurred or relatively distinct images.

(2) Distinct vision increases as object size decreases relative to visual targets of Landolt, requiring 1,000～1,500 msec in Landolt 1.0 (visual angle 1.0’). This seems to correspond with times of accommodation and fluctuations of accommodateon in daily life vision.