Problems concerning film flatness

As you may see on the picture, the upper and lower film edges tend to curl.




If the upper and lower edges of the film are pressed down, how will the center area react? The center will come up. This is the actual situation in Pentacon Six cameras:

On the drawings we look along the film from spool to spool, film rails are blue. The pressure plate is pressing film and backing paper (which are combined here in just one black line) onto the film rails, film center area bending towards the lens.




This problem increases if backing papers do not have a perfectly even surface (what I found at the former Forte films). The 220 type is better in this respect of course.

Modern medium format cameras have pressure plates with a certain space given for film and backing paper which is a bit more than just their thickness. This is achieved by an additional pair of film rails which the pressure plate rests on. The baierfoto solution for Pentacon Six has such a device. Also the film rails are milled to a narrower size to give more space for the edges of the film:

The weakness of the original pressure plate of Pentacon Six adds to this effect - the plate itself bends towards the lens in its center. A laid on ruler makes this visible:

As a simple solution is to bend the plate in the opposite direction. Some bend in the opposite direction should remain, so that the plate will lie flat under spring tension with the camera back closed. This is the baierfoto Economy solution (page in German only, sorry). In the baierfoto main solution (page in German only, sorry) however, the original pressure is replaced by a pressure plate the spring of which does not press in the center.

Adjustment of the focusing screen in Pentacon Six wih original pressure plate:

The adjustment has to take into account that the film surface is about 0.2mm nearer to the lens than it should be. Therefore with a Biometar 2.8/80 standard lens the infinity focus should be reached near the 30 meter mark on the scale. For certain film brands a special fine adjustment might become necessary.






Flatness with Kiev 60 and Exakta 66



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