Purpose: Contrast constancy describes the well known observation that the perceived contrast of supra-threshold gratings is approximately independent of spatial frequency despite differences in threshold. We examined the temporal dependence of perceived contrast and the effects of retinal blur to explore the underlying mechanisms of contrast constancy. Method: In a forced choice procedure, subjects compared the contrasts of sine gratings varying in spatial frequency (1-16 c/deg), physical contrast (10%-60%), retinal contrast (1-5 fold reduction), and duration (13-430 msec, time parameter of the Gaussian spatio-temporal envelope). Results: Gratings of different frequency and equal physical contrast grew with exposure duration to equal perceived contrast. However, the rate of growth decreased with increasing spatial frequency. Degrading the retinal contrast of a grating increased detection threshold but had little effect on the final perceived contrast. Conclusions: 1) Contrast constancy requires time to develop; and 2) the visual system has a mechanism that allows rapid recalibration of constancy.