There is provided an operating method of an image sensor including: storing a first charge from an optoelectronic circuit to a pixel buffer circuit within a first exposure period; storing a second charge from the optoelectronic circuit to the pixel buffer circuit within a second exposure period; transferring the first charge from the pixel buffer circuit to a first storage circuit outside of pixel circuits; transferring the second charge from the pixel buffer circuit to a second storage circuit outside of pixel circuits; and comparing the first charge with the second charge to output an analog image signal.