Archived posting to the Leica Users Group, 1996/04/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]On Fri, 12 Apr 1996, Eric Welch wrote: > Paper backing allowes little windows on the film backs to show the > writing on the back of the film without fogging the film. And there is > less reason to have high-resolution and contast than 35mm film. That's a > function of emulsion, and I don't think that's different. Lack of flare > will make a difference, but I don't think base density would influece > those two factors. 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