An image sensor includes a pixel array connected to a plurality of column lines, a first ramp signal generator generating a first ramp signal, a second ramp signal generator generating a second ramp signal, and an analog-to-digital conversion (ADC) circuit operating in a first mode. The ADC circuit includes a first comparator group comparing the first ramp signal with a first pixel signal received from a first column line group, among the plurality of column lines, and a second comparator group comparing the second ramp signal with a second pixel signal received from a second column line group, among the plurality of column lines. The comparing of the first ramp signal occurs at a comparison time point different from a comparison time point during which the comparing of the second ramp signal occurs.