An image sensor includes a pixel array and a calibration circuit. The pixel array includes a plurality of pixels each of which includes a photoelectric conversion device configured to absorb incident light and generate a photocharge, a transfer transistor configured to transfer the photocharge from the photoelectric conversion device to a floating diffusion node, and a reset transistor configured to reset the floating diffusion node. The calibration circuit is connected to the reset transistor of each pixel, and is configured to apply a different voltage to each pixel and adjust an amount of photocharge generated by the photoelectric conversion device in each pixel.