Changes in ocular astigmatism over the whole range of accommodation.

The refraction of 20 subjects was measured over the whole range of accommodation using an objective automated refractometer in order to investigate the astigmatic change whose origin is presumed to be at the lens. For this purpose, the corneal vertex was used as the plane of reference to avoid changes in the optical effectively of a cylindrical lens at near. The change in the astigmatism with accommodation was observed in each subject. Subjects were then divided into three groups: (1) seven subjects showed an increase in astigmatism with accommodation; (2) eight subjects showed constant astigmatism up to the accommodative nearpoint; and (3) five subjects showed other types of change, such as reduced astigmatism in the intermediate accommodative state. These variations suggest that accommodative astigmatism is affected by many elements such as lens, zonule, and muscle. Two of 20 subjects could see the vertical line of the visual target composed of many directional lines clearer than the horizontal line at distance. Conversely, they could see the horizontal line clearer than the vertical line at near. We also studied the correspondence between the dark focus of accommodation and the dioptric distance where astigmatism is at a minimum, as suggested previously. The results showed that no such direct relation existed.