If f/2.8 is not fast enough for you, here's an f/2 lens. The focal length is about 47 mm.   The lens is not yet optimized for "buildability". Also, the distance from the rear element to the sensor required for infinity focus is less than that of lens #2. Mounting a helicoid would be more difficult.   The lens does not correct for chromatic aberration.   Lens simulated using WinLens3D Basic. Simulated at 340 nm.