Diffraction limit. Arash posted a lint to Cambridge color:
http://www.cambridgeincolour.com/tut...hotography.htm
but the premise of the page and their definition of diffraction limit is wrong and does not even make sense:
"at some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field. When this occurs your camera optics are said to have become
diffraction limited."
That premise is incorrect. One can often have a depth of field that is very low and smaller apertures, even beyond the Dawes limit will still improve the depth of field and improve the sharpness of an out of focus area, especially in macro photography.
Diffraction limit is when the optical system resolves all the detail in an image (samples the Dawes limit; MTF=0). Diffraction limit has nothing to do with out of focus things in an image. (I will contact Cambridge color about their web page).