Astigmatism, accommodation, and visual instrumentation

The possibility is discussed that, through the exercise of his accommodation, an observer may be able to compensate partially for the aberrations of a visual instrument, in particular, for field curvature and astigmatism. The relevant properties of the accommodation response are briefly reviewed. Experiments are described in which an astigmatic image field was simulated with the aid of cylindrical lenses, and the accommodation responses were determined with a laser optometer. Observers were found to adjust their accommodation with the orientation of a grating target in such a way as to attempt to minimize the effects of the instrumental astigmatism. The degree of compensation achieved depended upon the position of the image vergence within the range of accommodation of the observer. Field studies with a fixed-focus sighting telescope support these ideas.