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

[kcarney1 at cox.net (Ken Carney)]

I think it is just something to verify with each lens.  Before digital 
and 35mm cameras, most of my photo experience was with larger format 
cameras (4x5" and 8x10" negative size).  Even there, stopping down to 
f/64 was problematic sometimes.  The saving grace, as noted, was that I 
enlarged to at most 16x20" and the 8x10's were mostly contact prints.  
"Sharpening" did not exist, except for the painstaking procedure of 
making unsharp mask negatives for enlarging.  As an example, I have a 
Canon 70-200 2.8 IS lens.  It is extremely sharp, but a few days ago I 
accidentally took some images that turned out at f/32 and f/45.  They 
were unusable.  So, I don't have too much to add on a simple explanation 
except for, don't stop down that sucker too much (I seldom go beyond f/8 
to control DOF).

Ken

On 11/27/2010 6:11 PM, afirkin at afirkin.com wrote:
> 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.
>
>
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