Lens Diffraction : does stopping down a lens make it sharper or not ?

"Every photographer wants both maximum resolution and maximum depth of field. But unfortunately these two demands can be mutually exclusive. As you stop down the aperture on a lens the light passing through tends to diffract, reducing sharpness, though DOF is increased. The reason for this is that the edges of the diaphragm blades in the lens tend to disperse the light."



