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. In the center of the image the film surface (black) is nearer at the lens than it should be (green line). The weakness of the original pressure plate adds to this effect - the plate itself bends towards the lens in its center.

Let us now have a look at the situation in cameras with an additional pair of rails (or other device) to support the pressure plate at a definite distance from the film guiding ones.

In the Kiev 60 we have a free space for film and paper of 0,5mm, which is more than film´s plus paper´s thickness of about max. 0,3mm. Therefore the film edges have much space and are not pressed, film lying flat on the pressure plate. But, the film´s distance from the lens is rather too big, so that in some cases a piece of self adhesive sheet material of 0,1 mm thickness sticked onto the plate is recommendable, upper and lower edge of the plate have to remain free, of course, otherwise the whole plate would be lifted.

In the Exakta 66 we have a similar construction, only that the space for the film (and paper in case of 120 type film) is not sufficient. Pieces of self adhesive sheet material of 0,1 mm thickness sticked on the plate´s lashes can help and improve flatness and distance to the lens as well.

In case of the Pentacon Six a certain improvement is possible by carefully pre-bending the pressure plate in the opposite direction, so that is lies flat under its spring´s tension. Also the focusing screen´s adjustment has to be done according to the film´s actual position.

For Pentacon Six and Exakta 66 an upgrade solution ist available (currently german page only).

The film flatness is, by the way, influenced by the film brand, too. Modern western films differ from eastern films. E.g. efke and Forte bend up to 0,2 mm higher in the center!