Re: DOF and diffraction

[P3D john bercovitz <bercov@xxxxxxxxxxx>]

John O writes:
> WRT to small apertures, I believe it was John B. who explained earlier
> that if you encounter a loss of resolution as you decrease the aperture,
> then you may be experiencing refraction effects which are dependent on
> the physical size of the aperture, not the f/ number.  Thus a wide
> angle, normal view, and telephoto all of the same quality would tend to
> degrade when the aperture has a certain diameter, not a certain
> f/number, hence each lens would degrade at different f/numbers, the wide
> angle degrading at a lower f/ number than the normal and telephoto.
> 
> Did I get that right?
 
Very close - the word you wanted there was diffraction rather than 
refraction.  And yes, diffraction effects do increase with decreasing 
linear or absolute aperture (as opposed to relative aperture, often 
called f/number).  Because people view stereo pairs from the center 
of perspective, and because angular resolution is independent of focal 
length or f/number and is strictly dependent on lens opening, only angular 
resolution matters to a stereophotographer (and not plate resolution).
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So if you want to stay away from diffraction problems, switch to a larger 
format and suffer depth of field problems instead.  8-)  Seriously, we
don't often get into diffraction problems at common lens apertures.  
Lemme do a quick calc... please?  8-)  
 
This probably looks familiar to anyone who's done any telescope work.
Using the Rayleigh criterion,
R = 115/D 
where:
D = diameter of entrance pupil in millimeters
R = Angular resolution in seconds of arc
 
The resolution of the eye is about 60 seconds of arc under good conditions.
So our minimum D is
 
D = 115/R = 115/60 = 1.9 mm
 
So now we know we want to have an aperture of at least 1.9 mm.  What does 
this mean in relative aperture terms?
 
Lens focal length     f/number for 1.9 mm aperture
 
      35                        18
      50                        26
      80                        42
     150                        79
 
Diffraction becomes a contributor as you approach these f/numbers and 
it is a major or minor contributor according to how sharp the scene is 
otherwise. 
 
Also because stereophotography concerns itself with angular resolution 
and not plate resolution, film grain goes away with finer films in medium 
format but will be visible to very sharp eyes in 35 mm format.  This is 
because the grain size is fixed but you view it from farther away in 
medium format.
 
John B
 
 


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