Thanks --Michael [EMAIL PROTECTED] for the up-to-date data. My source is ancient and lacks tolerances (I think!) However, I question one detail below:
> The ANSI standard for the depth of a standard 4 x 5 inch film >holder is 0.197" plus minus 0.007". Most film has a base > of 0.007" . When film is loaded in the film holder, the depth is >0.190". This is the measurement used by Sinar cameras. > Wisner cameras use a compromise of 0.192" to allow for wear on the >wood and because Tech Pan film, used by some > photographers to achieve ultra-sharp images, has a base of 0.004". > >############### I infer from this that you must add a correction >depending on the thickness of the film base but I believe this is wrong, >at least for the film holders I know. The location of the film plane is >set by a narrow ridge around the edge of the film. The film is pressed >forward against this fixed ridge by a movable pressure plate until it >contacts the fixed rige. It is the emulsion which is then at the >location of the ridge surface and the location will be independent of the >base thickness. You might consider a correction for emulsion thickness >but that will be far below the tolerance band except for some unusual >film, and even with such a film I might select forming a sharp image at >the surface, where diffusion in the emulsion has not yet taken its toll. >The only time one should even begin considering the base thickness is with >a film lacking an anti-halation backing or tinted base, both of which >surpress reflections from the back of the film. Or, one other special >case, the vacuum back. Here it depends on how the film is located. If >the film still presses against that ridge and the vacuum back floats to >conform to the ridge location, nothing will be changed. In the big repro >cameras I have seen in use, the back is fixed and arbitrary size film is >sucked against the plate. The location of the back does not change to >accomodate the film thickness and here a correction will be needed. As for the tolerance band, I would try to stay closer to the 0.190; if my "back-of-the-envelope" calculation (based on simple geometrical considerations) an error of 0.007 inch will degrade the resolution considerably at F4, and if you are lucky enough to have an F2.8 lens for 4x5 (Aero Ektar or Schneider Xenon, at 2.8) and wish to photograph at full aperture, you will be reaching much less than the capabilities of the lens (50 lp/mm or better, and with the Aero Ektar, I believe well over 100 lp/mm, so the film is limiting if you focus well, which you can hardly do with sloppy 0.00 inch errors!) Bob Bob _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
