Ok, so you know that Medium Format means bigger film, right? That's pretty easy. But what about all this 6x6, 6x7, 120/220 business that you've seen along with that nebulous term, medium format? Read on.
You can see from the picture at right the obvious difference between 35mm and medium format. Unlike 35mm which is packaged in a cassette, medium format film is simply rolled onto a spool. While 35mm film is 24mm wide (or high, depending on how you look at it), medium format is 61mm wide, although the actual picture is somewhere between 54-57mm since medium format film has no sprockets like 35mm, so the actual picture size is determined by the camera. And while 35mm cameras use a 36mm length, medium format cameras employ various lengths of the film for the picture. There are 6x4.5, 6x6, 6x7, 6x8, 6x9, 6x12, and 6x17 cameras and all of those formats are compatible with either 120 or 220 film.
So just what does 120/220 mean, anyway? Well, they are basically the same. If you are using a 6x6 camera with 120 film you'll get 12 exposures and with 220 film double that. The film itself is exactly the same. That's simple enough. However, to your camera there's a big difference.
The 120 films come with a paper backing to the film while the 220's don't. That little paper changes where the film resides in the camera. With 120, the film would be closer to the lens, and with 220 further away. This difference means cameras either have to have a different film back for 120 and 220 to maintain the same focus, or they have to have an adjustable film pressure plate. For cameras with different film backs such as Hasselblad this is not so trivial, since each film back costs around $800. People usually choose what type of film they'll use and stick with it. Cameras with an adjustable pressure plate, however, such as the Yashica 124G, are much cheaper and simpler to operate. Simply slide the pressure plate to alternate between using 120 or 220.
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