An image sensor includes a pixel array including first pixels and second pixels, each of the first and second pixels including photodiodes, a sampling circuit detecting a reset voltage and a pixel voltage from the first and second pixels and generating an analog signal, an analog-to-digital converter image data from the analog signal, and a signal processing circuit generating an image using the image data. Each of the first pixels includes a first conductivity-type well separating the photodiodes and having impurities of a first conductivity-type. The photodiodes have impurities of a second conductivity-type different from the first conductivity-type. Each of the second pixels includes a second conductivity-type well separating the photodiodes and having impurities of the second conductivity-type different from the first conductivity-type. A potential level of the second conductivity-type well is higher than a potential level of the first conductivity-type well.