An image sensor includes a pixel having an internal capacitor. Each of a plurality of pixels of the image sensor includes a photodetection circuit and an analog-to-digital converter (ADC). The photodetection circuit generates a detection signal. The ADC converts the detection signal using a ramp signal. The photodetection circuit includes a photodiode, a floating diffusion node and an overflow transistor. The floating diffusion node accumulates photocharges generated by the photodiode and includes a parasitic capacitor. The overflow transistor electrically connects the floating diffusion node to a first internal capacitor of the ADC.