220 film is the same as 120, save and except:
1) 220 has an imaging area that is twice as long as 120;
2) 220 has a paper leader and paper trailer, and no paper behind the imaging area. 120 has one long piece of backing paper the entire length of the film, plus the leader and the trailer;
3) 220 backs backing paper has a start mark that is at a slightly different location than 120. As a result, you may end up having to use a slightly different start position if you run 120 through a 220 insert/back;
4) In the part of the film where the pictures go, there is no backing paper behind 220 film. So the backs for 220 film are designed to deal with a slightly thinner combination. As a result, Mamiya used to warn that using 120 film in a 220 insert/back may cause the insert/backs to wear more rapidly/require service and adjustment more frequently. I have no doubt that would only be relevant for high volume users;
5) of absolutely the most practical importance is the fact that a 220 insert/back has a different set-up for the frame counter. For 6x7, instead of stopping at 10, the 220 insert/back will merrily let you shoot the next ten pictures without any film in place - so extra care is critical!
The plane of focus is determined by the film gate, not the pressure plate, so focus doesn't change.
