An image sensor includes; a sensor array generating a pixel signal, a ramp signal generator generating a ramp signal having a decreasing slope during a ramp signal enable period between a first time at which a counter enable signal is activated and a third time at which the ramp signal ends falling, a comparator comparing the pixel signal with the ramp signal to trigger an output signal, and counters, where at least one of the counters performs counting during an entire counter enable period extending between the first time and a second time at which the comparator triggers the output signal, but not all of the counters perform counting during at least one section of the counter enable period.