An image sensor may include an image pixel array with both image pixels to gather image data and phase detection pixels to gather phase information. The image sensor may dynamically group the phase detection pixels into focus zones. Based on the characteristics of the scene being imaged and the use settings of the image sensor, the image sensor may determine the size, shape, and number of focus zones to be grouped. One or more focus zones may then be used to gather phase information data. The focus zones may vary in size and shape across the pixel array. A scene with low illumination level may result in larger focus zones with more phase detection pixels to ensure reliable phase information data.