An image sensor includes a pixel array, a plurality of comparators, a plurality of counters and a plurality of synchronization circuits. The pixel array includes a plurality of pixels configured to generate analog signals by sensing incident light. The comparators generate comparison signals by comparing the analog signals with a reference signal. The counters are grouped into a plurality of counter groups. Each of the counters generates digital signals corresponding to the analog signals by counting, the counting terminated by the comparison signals. Each of the synchronization circuits synchronizes input clock signals to a source clock signal to provide synchronized input clock signals to each of the counter groups.