Archived posting to the Leica Users Group, 1996/04/12
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Pardon me: I should have written "non-reflective" backing!
Gary ^^^^^^^^^^^^^^
On Fri, 12 Apr 1996, Gary J Toop wrote:
> I believe that film-base density has two effects on sharpness
> and fog. Since base is not completely transparent, it does very slightly
> limit the amount of light which passes through it and, the thicker the
> base, the more it does this. This may have the effect of somewhat
> mitgating the other effect of film base thickness: the distance between
> the emulsion and the reflective backing permits some diffraction of light
^^^^^^^^^^
> particles as they bounce off the backing and back through the emulsion.
> The greater the distance, the greater the amount of diffraction. Thus, a
> thinner film base will have two effects: it will decrease the amount of
> diffraction, improving sharpness at least a little, and it will permit
> the direct passage of a little more light. This latter effect may be
> more significant if you are using a condenser rather than a diffusion
> enlarger, although I am not sure. The backing used with 120 film permits
> the use of a thinner film base, I believe.
>
> So, unless someone else out there has some reason to believe that
> that account is not correct, there is probably at least a slight
> difference between the sharpness of 35mm and 120 films which is due to
> the difference in thickness of their respective film bases.
>
>
> Hope this clear up some things.
> Gary Toop
>
>