Archived posting to the Leica Users Group, 1996/04/19

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To: <>
Subject: Diffraction and thin film
From: Charles E. Dunlap <>
Date: Fri, 19 Apr 96 12:16:53 -0700

>I don't see how diffraction happens in film. Diffraction happens when 
>light bends around a sharp edge at a steep angle, like when the aperture 
>is set to a small size. You mean refraction?
>As far as it goes, 120 film can be "sharper" because the film base is 
>smaller, I haven't seen anything to say one way or the other, so I'll 
>take your word for it. BUT, MF lenses aren't as sharp as 35mm lenses, 
>because they don't have to be. Maybe Zeiss and Schneider lenses are, but 
> you're wasting contrast to have such high resolution with the 
>enlargements that are going to happen.
>Eric Welch
>Grants Pass, OR

I'm not sure exactly what is meant by diffraction in film either. 
Diffraction in general occurs when light passes through a gap between two 
edges, this can be a straight slit, a hole (aperture), or an irregular 
gap: monochromatic light shining through an old razor blade will produce 
diffraction fringes. So, perhaps if diffraction is occuring in film it's 
due to light passing through gaps between bits of the emulsion or to 
linear strands of whatever organic molecule the film base is made out of. 
Alternatively, people could be speaking of diffraction in a lay sense and 
simply meaning to say that the light gets scattered in the emulsion by 
bumping into whatever's there. In either case, Fuji, in their press 
releases concerning their new APS films, make a point of saying that much 
of the improvement in sharpness and contrast is due to a new 20% thinner 
film base that they have developed for the APS films. Tech Pan uses a 
thinner base than other black and white films, so empirically at least 
thinner is better.


Charles E. Dunlap
Earth Sciences Dept.
University of California
Santa Cruz, CA 95064