A pixel array included in an auto-focus image sensor includes a substrate, a plurality of pixels, a deep device isolation region and a plurality of first ground regions. The substrate includes a first surface on which a gate electrode is disposed and a second surface opposite to the first surface. The plurality of pixels are disposed in the substrate, and include a plurality of first pixels configured to detect a phase difference and a plurality of second pixels configured to detect an image. The deep device isolation region is disposed in the substrate, extends substantially vertically from the second surface of the substrate to isolate the plurality of pixels from each other. The plurality of first ground regions are disposed adjacent to the first surface in the substrate and adjacent to only at least some of the plurality of first pixels.