Time course and variability of dark focus.

When the eye is deprived suddenly of visual stimulation, the accommodative system drifts from the previous state of accommodation to a state known as the dark focus. This condition also is known as night myopia. We measured the time course of this drift using a continuously recording infra-red optometer. The time course resembles an exponential decay function with a time constant of 1-3 seconds. The recovery of accommodation to the prior level after visual stimulation is restored suddenly has a time constant of 0.2-0.4 seconds. The state of accommodation in the dark depends on the state of accommodation prior to the onset of darkness. Our subjects showed a zone of accommodative inactivity rather than a single resting point of accommodation.