An imaging system with an array of pixel cell units having four photodiodes sharing the same readout amplifier, row select transistor, microlens and filter color. Some pixel cell units provide a phase detection autofocus function. The imaging system provides the option of imaging with phase detection autofocus and with or without horizontal or vertical binning readout modes, or both simultaneously, to form an a full resolution image of the target scene with improved effective frame rate and improved slow motion imaging.