Image-focusing method and associated image sensor

An autofocusing method includes capturing an image of a scene with a camera that includes a pixel array; computing a horizontal-difference image, and a vertical-difference image; and combining the horizontal-difference image and the vertical-difference image to yield a combined image. The method also includes determining, from the combined image and the intensity image, an image distance with respect to a lens of the camera at which the camera forms an in-focus image. The pixel array includes horizontally-adjacent pixel pairs and vertically-adjacent pixel pairs each located beneath a respective microlens. The horizontal-difference image includes, for each horizontally-adjacent pixel pair, a derived pixel value that is an increasing function of a difference between pixel values generated by the horizontally-adjacent pixel pair. The vertical-difference image includes, for each vertically-adjacent pixel pair, a derived pixel value that is an increasing function of a difference between pixel values generated by the vertically-adjacent pixel pair.