Originally posted by Nesster Actually if you think about it - there is absolutely no difference in registration distance - the film is pressed against the film rails, flat, in the focal plane. The difference, if any, might be in the spring tension on the back plate that presses the film to the rails.
Almost right. The real answer has to do with the fact that 120 film has the paper backer behind the film as it passes the backing plate; it's thicker. With 220 film, the paper ends as the film begins - i.e., the paper lead and tail is taped to the ends of the film. Consider that they must get twice as much film on the same size spool so the paper ends where the film begins.
It's this thickness that accounts for the difference in the back plate dimensions. Given that every manufacturer provides both film holders there must be a good reason.
I'm going back about 35 years to Mamiya 6x6 TLRs and something may have changed since, but that's how I learned it. As I recall, cheatin' on film holders caused the same sort of frustration as auto focus issues do today.
H2
I just thought to Google "220 film thickness" and found this is a common question for people new to ML film.