Distinct subclasses of retinal ganglion cells (RGCs) mediate vision and nonimage-forming functions such as circadian photoentrainment. This distinction stems from studies that ablated melanopsin-expressing intrinsically photosensitive RGCs (ipRGCs) and showed deficits in nonimage-forming behaviors, but not image vision. However, we show that the ON alpha RGC, a conventional RGC type, is intrinsically photosensitive in mammals. In addition to their classical response to fast changes in contrast through rod/cone signaling, melanopsin expression allows ON alpha RGCs to signal prior light exposure and environmental luminance over long periods of time. Consistent with the high contrast sensitivity of ON alpha RGCs, mice lacking either melanopsin or ON alpha RGCs have behavioral deficits in contrast sensitivity. These findings indicate a surprising role for melanopsin and ipRGCs in vision.