[Leica] Does anyone have a simple way to explain this

[afirkin at afirkin.com (afirkin at afirkin.com)]

Diffraction Limits:

The diffraction limit is dependent on the wavelength of the light and the
f-stop.   It is calculated as the size of an Airy disk (Sir George Airy). 
 This describes the size of a circle from a point of light as it passes
the edges of the aperture.   Fundamentally, as you decrease the aperture
size (increase the f-stop) the size of the Airy disks increases.   An Airy
disk can also be called a blur circle at the image plane similar to the
CoC for DOF.   The simplified calculation (estimation) is 1500 divided by
the aperture.   Thus, f/8 yields .005 mm and f/64 yields .042 mm.   The
relationship between the size of the Airy disk and resolution is known as
the Rayleigh criteria.   Since it takes two of these points of light to
define resolution the respective resolutions are 100 lp/mm for f/8 and 12
lp/mm for f/64.   Obviously, stopping down the aperture to increase depth
of field can lead to increased diffraction that blurs the entire image.

Most photographers would agree that 35mm cameras become diffraction
limited at f/22.   At smaller aperture sizes diffraction trumps depth of
field and the overall image loses resolution.   For a typical digital
sensor size the diffraction limit is a factor at f/16.   Diffraction
limits are less significant with medium and large format cameras.   This
is only because there is less need for enlargement therefore the CoC
values can be relaxed.