Re: [Leica] RE: soft focus vs out -of- focus in an ultimate sharpness world

["Patrick G. Sobalvarro" <pgs@sobalvarro.org>]

At 08:17 AM 12/3/97 -0600, Eric Welch wrote:
>At 12:04 AM 12/3/97 -0800, you wrote:
>
>>died; it just went underground when the f/64 guys (who don't seem to have
>>known about diffraction) beat up on it.  It lives on in the hearts of the
>
>Actually, they know very well about diffraction. But it's not a big deal
>with large format lenses. They tend to be longer in focal length, and the
>longer the focal length, the smaller the aperture has to be for diffraction
>to be trouble.

Actually, the f/64 guys (Weston, Adams, Van Dyke, and their followers, who
in rebelling against pictorialism formed the f/64 school) don't know
anything very well.  They're dead.  But even when they were alive, they
couldn't have known very much about diffraction, or they wouldn't have been
under the impression that the name "f/64" would connote ultimate sharpness.

Your claim that diffraction is less of a problem with longer focal lengths
is incorrect.  The limit of resolution (as measured by the Rayleigh
criterion) of a diffraction-limited lens is given by

r = 1.22 L f / D

for L the wavelength of light, f the focal length and D the diameter of the
circular aperture.  You will recognize f / D as the f-number.  So f/64 in a
diffraction-limited lens produces a limit of resolution of about .04 mm
(taking the wavelength as ~500 nm), irrespective of the focal length of the
lens.

This is good news for you.  It means that the concern you raise when you say

>The problem is for us who use wide lenses in 35mm. Like my 19mm 2.8.
>Diffraction can be quite severe at f/22. I'll have to check it.

shouldn't worry you too much.

- -Patrick