Image sensors and methods of operating the same. An image sensor includes a pixel array including a plurality of pixels. Each of the plurality of pixels includes a photo sensor, the voltage-current characteristics of which vary according to energy of incident light, and that generates a sense current determined by the energy of the incident light; a reset unit that is activated to generate a reference current, according to a reset signal for resetting at least one of the plurality of pixels; and a conversion unit that converts the sense current and the reference current into a sense voltage and a reference voltage, respectively.