The "definitive" answer would be the result of running 120 and 220 film through the same 220 film back, shooting the same scene, and everything else the same too.Try it with a tripod-mounted camera, shooting the same scene on the same film (one in 120 and one in 220) and inspect under a loupe to see if there is a difference with your camera, if you want to know the definitive answer.
It depends on the design of the back. I can't answer for RB67 backs, but usually the film plane is determined by rails on the emulsion side of the film, not the pressure plate, and not all medium format film base is the same thickness in any case, so the backing on 120 film shouldn't cause a focus problem in a 220 back, but there could be other problems like excessive film tension causing more wear in the back or even the risk of the leader tearing in backs with particularly high tension.
On the other hand, 220 film in a 120 back is likely to have flatness problems, depending on the style of pressure plate, or may be at greater risk for scratches on the base side, because the pressure plate in a 120 back doesn't have to be as smooth as the pressure plate in a 220 back, and then you may lose a frame somewhere, because the frame counter is designed for 120. In a Noblex medium format camera, for instance, you can use 220 per manufacturer's recommendation, but you lose a frame or two in the counter restart. Some backs may not let you restart without opening the back.
Zeiss published an article in Zeiss Lens News a few years ago claiming that 220 film in 220 backs that they tested generally had better film flatness than 120 film in 120 backs. I don't think they tested RB67 backs, because they don't make RB lenses, but it was an interesting observation.
Which is my cue for pointing out that Zeiss praised 220 film the moment a vaccum back was launched for 'their' Contax camera.Zeiss published an article in Zeiss Lens News a few years ago claiming that 220 film in 220 backs that they tested generally had better film flatness than 120 film in 120 backs. I don't think they tested RB67 backs, because they don't make RB lenses, but it was an interesting observation.
Which is my cue for pointing out that Zeiss praised 220 film the moment a vaccum back was launched for 'their' Contax camera.
Vacuum backs do not work when there is paper between film and suction plate.
The department that produced that article was called "Strategic marketing".
That also is an interesting observation.
Indeed it is, but they also said the same was true for Hasselblad and Alpa, for which they also make lenses, and the Alba back is actually made by Linhof, except for the mounting plate.
I'm glad we could help. NOW GO OUTSIDE AND TAKE PICTURES!!!Marvellous! 120 it is then! I was worried about focus and the leader tape being broken and damging the back, but the advice is brill! Thanks for all the advice!
That's correct generally speaking David, but just to add a side note the RB67's pressure plate has 2-3 mm something of clearance on the pressure plate. Things MUCH MUCH thicker than a film + paper back could pass through the rails and pressure plate assembly. Probably even a glass plate would fit in there. Paper backing's thickness is simply NOTHING compared to the clearance the pressure plate has, and that's why it works so well.It depends on the design of the back. I can't answer for RB67 backs, but usually the film plane is determined by rails on the emulsion side of the film, not the pressure plate, and not all medium format film base is the same thickness in any case, so the backing on 120 film shouldn't cause a focus problem in a 220 back, but there could be other problems like excessive film tension causing more wear in the back or even the risk of the leader tearing in backs with particularly high tension.
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