It will illuminate 12x20 at f9. Cover adequately, that's a different question. The diagonal of the 16x16 is about the same as the 12x20 diagonal, so if it does cover 12x20, it will cover 16x16 as well. But I think you'd be pushing it for adequate sharpness in the corners wide open. Stop down to f22 or f32 and you'll be good to go, though.