An infrared optometer, which is capable of continuously measuring the refractive power changes in the human eye, and a target display system with variable artificial pupil have been devised. Using these devices, eye accommodation has been observed and the differences of the accommodative responses among various effective pupil diameters have been investigated.
On steady state accommodation, it has been observed that the refractive power decreased as the pupil size decreased. Amounts of the decreases were measured for three subjects and it was made clear that they were unrelated to the magnitudes of accommodative stimuli and the luminance of the target. It was found that the amounts depended only on the pupil size and were represented by a reverse function of the pupil diameter. Further, it was found that they were closely related to the half values of the depth of focus of the eye obtained by K. N. Ogle. Therefore, a hypothesis has been deduced that at steady state, visual point is placed on the position farther than the object with an offset, which is equal to a half of the depth of focus, so as to produce the minimum detectable amount of blur on the retina. The size of the minimum detectable blur has been estimated as 24 At on the retina from the observed accommodation offsets.
In order to verify the hypothesis, transient responses of accommodation to stepwise target motion were investigated. When the target moved from near to far, very slow responses (response time ranging within 2-5 sec) were often observed for small target motion. Quantitative investigation revealed that this kind of response was caused by the depth of focus, and the eye can accommodate stably on only the farther point among the two points where the critical amount of blur was produced on the retina.
Finally, in human eye accommodation, a control algorithm for steady state accommodation is considered to be used so that the eye increases its refractive power when the blur on the retina exceeds the threshold value and decreases when the blur is below the threshold.