An analog-to-digital converter (ADC) within an image sensor includes a comparator comparing a ramp signal with an image signal, and a counter generating a count result in response to the comparison by counting a clock during a counting interval. The ADC determines whether a first counting interval for the counter is less than a reference interval, and if the first counting interval is less than the reference interval the counting interval is a first counting interval, else the counting interval is a second counting interval.