And the F stop is a fraction of the focal length. f16 on a 500mm lens is a whole lot larger than f16 on a 50mm lens. It's the relation of the size of the hole to the wavelength of light going through it that causes diffraction.
Really? I'll be damned. I always thought it was the ratio of light diffracted by the edges of the diaphragm to the undiffracted light coming through the clear aperture. I'll go back to physics 101 I guess.