Each pixel on the CMOS imaging sensor has two separate, light-sensitive photodiodes, which convert light into an electronic signal. Independently, each half of a pixel detects light through separate micro lenses, atop each pixel. During AF detection, the two halves of each pixel -- the two photodiodes -- send separate signals, which are analyzed for focus information. Then, an instant later when an actual image or video frame is recorded, the two separate signals from each pixel are combined into one single one, for image capturing purposes.