Acts as a calcium sensor and regulates phototransduction of cone and rod photoreceptor cells. Modulates light sensitivity of cone photoreceptor in dark and dim conditions . In response to high Ca2+ levels induced by low light levels, prolongs RHO/rhodopsin activation in rod photoreceptor cells by binding to and inhibiting GRK1-mediated phosphorylation of RHO/rhodopsin. Plays a role in scotopic vision/enhances vision in dim light by enhancing signal transfer between rod photoreceptors and rod bipolar cells. Improves rod photoreceptor sensitivity in dim light and mediates response of rod photoreceptors to facilitate detection of change and motion in bright light.