Steady state of accommodation during observation of a Scheiner image.

To evaluate the observer's ability to self-monitor and correct accommodation voluntarily, a slit aperture, incorporating the Scheiner principle, was used as an accommodative stimulus. The Scheiner image consisted of two horizontal lines, which were perceived as superimposed into a single line when the observer's accommodation was conjugate with the optical distance of the target. The observer's task was to maintain superimposition of the monocularly viewed lines. During the task, accommodation was assessed with a Badal optometer that incorporated gratings of several spatial frequencies. By varying the optical distance of these gratings it was possible to determine the location of minimum blur, and thus the accommodation response. Results from two observers indicated that a Scheiner image can be used effectively for voluntary adjustments in accommodation for several optical distances.