This article was written by Michael Reichmann and originally published Jan 13 2009

 
This is not a comprehensive treatise on the subject of lens diffraction effects. Rather, it's purpose is to bring to the attention of photographers who may not be aware of the issue that there is no free lunch when it comes to achieving greater depth of field though stopping down.

Every photographer wants both maximum resolution and maximum depthof field. But unfortunately these two demands can be mutually exclusive. Asyou 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 theedges of the diaphragm blades in the lens tend to disperse the light. At largerapertures this diffracted light is only a small percentage of the total amountof light hitting the sensor or film, but as the aperture is stopped down theamount of diffracted light becomes a larger percentage of the total amount oflight being recorded...