[Leica] Lens Not sharp @ f16

[Malcolm McCullough <MM4@mm-croy.mottmac.com>]

In 35 mm still photography diffraction can become noticeable at f11, never 
mind f16.

John Brownlow wrote the truth:
>More precisely, if anyone cares, the effects of diffraction can at varioius
>apertures can be related to the size of the circle of confusion and the film
>format.


What is a bit odd is that in this case (light source distant from the 
aperture = Fraunhofer diffraction)  the size of the ceentral disk of light 
created by diffraction is dependent only on relative aperture. It does not 
depend on aperture - film distance. Weird, huh? The magic of diffraction.

If anyone is interested, the relevant formula is 
d = 2 * wavelength * fstop

where 
d is the diameter of the bright portion of the disk in the middle of the 
diffraction pattern
(if you want the diameter of the first dark ring outside the disk it is 2.44 
* wavelength * fstop)
d and wavelength are in the same units ( for mm use wavelength = 0.0007)


>- -- In 35mm, f/16 is the stop at which diffraction effects start to become
>visible (bigger than the standard allowable circle of confusion).


This (f16) sets a limit of about 45 lppmm. Some Leicaholic playmates may 
wish to aim higher for some kinds of work (cf posts on resolution by Erwin 
and others).
f8 would be the limiting stop to get 80 to 90 lpmm  (which is about the 
resolution of a 4000 dpi, ppi, or whatever you want to call it, scanner).
It's the same argument as with depth of field. One size of c-of-c does not 
fit all.

Regards,
Terence Tequinical
(The first 14 parts of my magnum opus, the "Fraunhofer Ring Cycle" will be 
published in Volumes 38, 45 and 303 of the Proc. Snake Oil Sales. Soc. Gr. 
Brit.)