Short-term variability of the resting focus of accommodation.

Previous investigations have shown that, in dim illumination and empty visual fields, accommodation in most observers is for an intermediate distance referred to as the dark focus or resting focus of accommodation. Dynamic properties of the resting focus were examined in the present study with a high-speed infrared optometer. All subjects displayed fluctuations of accommodation in darkness that were primarily characterized by frequency components below 0.5 Hz. These fluctuations were substantially attenuated during cycloplegia or viewing a bright, high-contrast target. Both the mean resting-focus position and the magnitude of fluctuations varied from one subject to another and from day-to-day. A weak association was found between the mean a standard deviation of accommodation responses in the dark. The present findings suggest that the accommodation mechanism in most observers is somewhat unstable under degraded viewing conditions. In addition, the mean resting-focus value for a large sample of subjects was found to be lower for i.r. optometer measurements than for previous investigations employing a laser optometer.