Medium format refers to roll-film cameras whose image formats are larger than 35mm film (often significantly so). Top quality medium-format SLRs like the Hasselblad or TLRs like the Rolleiflex were the workhorses of professional photography until digital photography displaced them. The term medium format almost always implies 120 film, although 220 (double the length of 120) and 620 (essentially 120 on a skinnier spool) would also qualify. In contrast, large format implies a camera designed for sheet film of 4×5" or larger. The 4×6.5cm vest pocket format of some cameras for 127 film defines the smallest medium format, while due to the actual state of progress of medium format sensor engineering in 2013 digital imaging sensors sized 49.1×36.8mm (Kodak 50 megapixel) and 53.8×40.3mm (Dalsa 60 megapixel) are also reckognized as almost the effective frame size 41,5×56mm of the second smallest medium format 4.5×6cm on film.
The width of the 120/620/220 films is about 62 mm; but by convention this is rounded off to "6" (centimeters) in referring to the different images sizes that are possible. Medium-format image dimensions are determined by the camera, and not by the film itself. That is to say, a roll of 120 film could be used to expose images of 6x4.5cm, 6x6, 6x7, 6x9 or even 6x12 cm depending on the camera. Alternate film backs for Hasselblad cameras allow to use 70mm film with sprocket holes to enable loading longer film stripes for up to 500 exposures.
Because of medium format's greater image area, additional detail and smoother tones may be recorded when compared to 35mm. This is true even when the camera is a rather inexpensive model, such as a basic TLR or folding camera. As such, medium format may be a surprisingly affordable method for photographers to improve the technical quality of their film images. The larger image area also implies higher costs for film and processing, but some find this a useful discipline encouraging a more deliberate approach to photography.
120 and 620 are rollfilms using paper backing to protect them from exposure to light, and should be loaded in subdued light conditions. In simpler cameras, a red window is used to view rows of frame numbers corresponding to different image sizes, printed onto the backing paper. 120 and 620 are nearly identical except in the spools, and it is quite possible to re-spool 120 film onto a recycled 620 spool, if only the smaller spool size will fit in a particular camera. 220 film is much like 120 except instead of a paper backing, it has a paper leader. Omitting the paper backing behind the film itself permits double the usual number of exposures to be wound up onto the same spool size used for 120.
- "Respooling 120 film onto 620 spools for use in older cameras", illustrated guide, from Glenn Stewart (archived).