Measurements were made of the ability of observers, whose accommodation had been paralysed, to detect the small changes in contrast resulting from sinusoidal, oscillatory changes in the dioptric focus of test objects, as a function of the mean position of focus. Variables studied included the form and wavelength composition of the test object, the diameter of the entrance pupil of the eye and the temporal frequency of the focus change. The dioptric amplitudes of the thresholds found (approximately 0.1 D) were comparable with the normally observed amplitudes of the microfluctuations of the accommodation system of the eye. A maximum for the threshold change of focus was found to occur about a mean position of focus corresponding to the nominal "best-focus". Two threshold minima occurred symmetrically on either side of this position of optimal mean focus; the positions of these minima depended on the spatial frequency content of the test object and the pupil diameter. The results are interpreted in terms of the corresponding through-focus changes in optical modulation transfer of the eye and data on the sensitivity of the visual system to spatial and temporal modulation. For sinusoidal grating test objects, it appears that the results can qualitatively be explained if it is assumed that the detectable focus change is such that the ratio of the modulation change in the retinal image, produced by the focus change, to the modulation at the mean position of focus is constant. Results for targets of broad spatial bandwidth suggest that sensitivity to focus change may be dominated by spatial frequency components approximately 5 c/deg. The implications of the data for the understanding of the accommodation control system are briefly considered.