P3D Re: Optimum Sharpness

["William J. Carter, Ph.D." <wc@xxxxxxxxxxxx>]

Ken Luker writes that John Toeppen wrote:

>Spot size due to diffraction is driven by the formula:
>
>2.44 x wavelength x f#

Since the physical diameter of the f-stop varies with the focal length, I
doubt the validity of the above formula. 

Classically, the resolution of a lens ( by Rayleigh) is expressed as an
angle theta:  

theta = 1.22 x lambda / a
(answer in radians) 

where lambda= wavelength (about 5.6 x 10 -5 cm for white light)
and a= diameter of lens aperture 

>Since 50 lp/mm is about what you have with Ektachrome 50, I
>think that we are talking a marginal loss here.

The total resolution of a system can be determined by the following formula:

                        R1 x R2
          R (total) ---------------
                        R1 + R2

"R1" could, for instance, be the resolution of the film, and "R2" be the
resolution of the lens.


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End of PHOTO-3D Digest 2990
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