Light intensity is an important determinant of diverse physiological and behavioral responses within the non-image-forming visual system. Thresholds differ among various photic responses, namely control of circadian rhythms, vigilance state, activity level and pupil constriction, but the mechanisms that regulate photosensitivity are not known. Calbindin D28k (CalB) is a calcium-binding protein associated with light processing in the mammalian circadian clock. Loss-of-function studies indicate that CalB-deficient mice (CalB−/−) have deficits in their ability to entrain to light–dark cycles. To explore the role of CalB in modulating photosensitivity, thresholds for three behaviors mediated by the non-image-forming visual system (entrainment, masking and pupillary light reflex; PLR) were compared in CalB−/− and wildtype mice, and the localization of CalB protein in these circuits was examined in adult and juvenile mice. The results reveal a divergence in how CalB affects thresholds to photic cues among these responses. Entrainment and masking were 40- to 60-fold less sensitive in CalB−/− than in wildtype mice. On the other hand, the PLR in CalB−/− mice was 80- to 200-fold more sensitive. Though CalB is expressed in the retina and in brain circuits regulating entrainment we found no CalB expression in any component of the PLR pathway, namely the olivary pretectal nucleus, Edinger–Westphal nucleus and ciliary ganglion. The behavioral and anatomical data together suggest that, in normal animals, the retinal response to light is blunted in the presence of CalB, but responsiveness of the higher order processes that transduce afferent retinal input is enhanced.