Image-focusing method and associated image sensor

An image sensor comprising:

• a pixel array including (i) a plurality of horizontally-adjacent pixel pairs, each being beneath a respective one of a first plurality of microlenses, and (ii) a plurality of vertically-adjacent pixel pairs, each being beneath either (a) a respective one of the first plurality of microlenses or (b) a respective one of a second plurality of microlenses;
• a processor coupled to the pixel array;
• and
• a memory storing machine-readable instructions that, when executed by the processor, control the processor to:
• capture an image of a scene with a camera that includes the pixel array;
• compute a horizontal-difference image that includes, for each of the plurality of horizontally-adjacent pixel pairs, a first derived pixel value mapped to a location of the horizontally-adjacent pixel pair within the pixel array and being an increasing function of a difference between pixel values generated by each pixel of the horizontally-adjacent pixel pair;
• compute a vertical-difference image that includes, for each of the plurality of vertically-adjacent pixel pairs, a second derived pixel value mapped to a location of the vertically-adjacent pixel pair within the pixel array and being an increasing function of a difference between pixel values generated by each pixel of the vertically-adjacent pixel pair;
• combine the horizontal-difference image and the vertical-difference image to yield a combined image;
• determine, from the combined image, an image distance with respect to a lens of the camera at which the camera forms an in-focus image of at least part of the scene;
• and
• adjust a distance between the pixel array and the lens until the distance equals the image distance.