An image sensor in accordance with exemplary embodiments of the inventive concept may include a pixel sensor array which includes an active pixel sensor and an optical black pixel sensor; a first analog to digital converter configured to convert a first sensing signal, which is provided from the active pixel sensor, to a first digital signal; a second analog to digital converter configured to convert a second sensing signal, which is provided from the optical black pixel sensor, to a second digital signal; and an output buffer configured to temporarily store and output the first digital signal and the second digital signal, wherein a plurality of noise characteristics of the second analog to digital converter is different from a plurality of noise characteristics of the first analog to digital converter.