I know a lot of people have been using higher f stop numbers f11-F22 to 'maximise' depth of field but at wider angles its really unnecessary and massivly reduces sharpness.
Have a play with the aperture on this test programm and see which apertures turn the optically superb canon 24-70 2.8 II into a mushy 'toy' lens!
http://www.dpreview.com/lensreviews/canon-ef-24-70mm-f-2-8l-ii-usm/3
NB the same will happen with any lens once severe diffraction kicks in!
Have a play with the aperture on this test programm and see which apertures turn the optically superb canon 24-70 2.8 II into a mushy 'toy' lens!
http://www.dpreview.com/lensreviews/canon-ef-24-70mm-f-2-8l-ii-usm/3
NB the same will happen with any lens once severe diffraction kicks in!