-----Original Message-----
From: PCA Cala <PCACala@xxxxxxx>
>
>"Due to optical diffraction, there is a limit to what a lens can ideally
>resolve. This is worse at smaller apertures (larger f/stop numbers). This
>limit is;
>
>1781 lines per mm / f-number
>
I have seen several formulas for diffraction, and this is the only one that
disregards focal length. However, there may be some theoretical basis to
this. Shipman states that diffraction is determined by the Total edge/Total
area or 4/Diameter. My email program balks at the symbols to show the
intermediate formula. The (Ilford) Manual of Photography (a great book I
just found for $3 at Goodwill) gives a formula for the resolving power of
lenses that does disregard focal length except for close-up photography.
This is based on the diameter of the circle produced by a point of light.
This is called an "Airy disc". The base formula for a theoretically perfect
lens is 2.44x(wavelength of light)x(distance of the image from the
lens/effective diameter of the lens aperture). For all except close-ups,
they give 2.44x(wavelength)x(f number). Close-ups and enlarging increase the
(distance/diameter) part and increase the effect of diffraction so the
original formula applies. The bottom line is diffraction from small
apertures is worse in close-ups.
>For example, at f/16, any lens can theoretically do no better than 111
lines
>per mm. At f/11 it is 162 lines per mm. This is why my tests rarely go
above
>f/11, and why I tend to shoot at f/11 or less in my own photography."
>
>He said that about f/11, not I. I prefer to make aperture settings at
>whatever the scene requires.
>
As complicated as this is, that may be the best idea.
>Gary Reese
>Las Vegas, NV
>
Doug
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