Accommodation is measured by flashing into one eye for 0.05 second every 10 seconds a beam whose configuration indicates whether the subject’s eye is over-accommodated, under-accommodated, or correctly accommodated for the measuring level. The exposure of the measuring beam is short enough not to contaminate the results with accommodative responses to it. Changes in accommodation can thus be followed by a bracketing procedure.
The visual stimulus presented to the eye is an empty field, both a completely dark one and one with a bright central area without sharp contours. The eye responds to this kind of stimulation by a fluctuating level of accommodation, with an average level of a little over 1D and with peak-to-trough amplitude of the oscillations of up to 1D, the most prominent period of the fluctuations being about two minutes. Harmonic analysis reveals that, while individual accommodation/time curves show strong frequency bands, there are no characteristic frequencies either for an observer or for a stimulus situation.
GERALD WESTHEIMER, "Accommodation Measurements in Empty Visual Fields," J. Opt. Soc. Am. 47, 714-718 (1957)