An image sensor includes a pixel array, a controller, and a plurality of analog-to-digital converters. The pixel array includes a plurality of pixels coupled to column lines, respectively, and the plurality of pixels are configured to sense incident lights to generate analog signals through the column lines. The controller generate a conversion control signal that is configurable based on changes of at least one operational condition. The plurality of analog-to-digital converters are coupled to the column lines, respectively. The plurality of analog-to-digital converters perform a delta-sigma modulation and a digital filtering to convert the analog signals to digital signals. The plurality of analog-to-digital converters adjust a conversion gain internally in response to the conversion control signal.