A pixel array of an image sensor includes a plurality of pixel groups. Each pixel group includes a plurality of unit pixels adjacent to each other and respectively including photoelectric conversion elements disposed in a semiconductor substrate, a color filter shared by the plurality of unit pixels, and a plurality of microlenses disposed on the color filter and having sizes different from each other such that the plurality of microlenses respectively focus an incident light to the photoelectric conversion elements included in the plurality of unit pixels. Deviations of sensing sensitivity of unit pixels are reduced and quality of images captured by the image sensor is enhanced by adjusting sizes of microlenses.