An image sensor includes a pixel including a first floating diffusion and a second floating diffusion, generating a first pixel signal based on a quantity of charge of the first floating diffusion, and generating a second pixel signal based on the quantity of charge of the first floating diffusion and a quantity of charge of the second floating diffusion; a column line connected to the pixel and transmitting the first pixel signal or the second pixel signal; and a readout circuit connected to the column line and generating an image signal based on a plurality of comparison results including a first comparison result obtained by comparing the first pixel signal with a first reference signal, a second comparison result obtained by comparing the second pixel signal with a first reference signal, and a third comparison result obtained by comparing the second pixel signal with a second reference signal.