A loose definition of medium format would be a film size that is larger than 35mm but smaller than 5"x4" large format. Historically, medium format film, often referred to as "roll film", has been produced in a number of different film types including: 105, 117, 120, 220, and 620. Nowadays the only film type still in production is 120 film. So, for all intents and purposes, a medium format camera is any camera that uses 120 film.
120 film was first made by Kodak in 1901 for use in their Brownie #2 camera. The film is approximately 61mm high and between 820mm and 850mm long, the actual image area is around 56mm high and continuous along the length of the roll. The film is rolled on to a plastic spool and wrapped in black backing paper which is longer and slightly wider than the film itself. The backing paper protects the film from exposure to light and allows it to be loaded and unloaded in daylight.
Unlike 35mm film, which is re-wound back into its canister when the roll is finished, 120 film is wound on to a second take-up spool. When the roll is finished the entire film will have been wound from the original spool on to the take-up spool.
One of the cool things about 120 film is that it can be used to produce a wide variety of photo sizes, from 6 x 4.5cm to 6 x 24cm wide. Some cameras are even capable of producing images in more than one size. The number of shots available on a roll is dependent upon the width of the images shot on any particular camera. The wider the image the fewer the number of shots per roll. The size of the image made by a camera is often designated in the camera's model name; for example, a camera that produces 6 x 4.5 cm photos may be referred to as a 645.
120 film is used in a wide variety of camera types, including:
- Toy cameras
- Folding cameras
- Singles Lens Reflex cameras
- Twin Lens Reflex cameras
- Rangefinder cameras
- Pinhole cameras
- Roll film backs for large format cameras (particularly for panoramic formats)
The cheapest way to start in medium format shooting would be with a toy camera, such as the Holga 120 or Diana F+. I'm sure toy cameras can be fun to use and have become very popular but they produce low quality, unpredictable images - which is the main appeal I guess.
Vintage folding cameras can be found at low prices but can be difficult to use and may have issues.
I suggest the best place to start would be with either a 645 camera or a TLR. You can achieve wonderful image quality without having to over-spend. If you invest in a reasonably popular model, and decide to upgrade later, you will recover your investment and may even make a small profit - if you buy well. The price of SLR cameras such as Mamiya RB/RZ, Hasselblad, or Pentax 67 have increased dramatically in recent years but will still be cheaper than a quality digital camera. Like anything, the more highly sought after the model, the more you have to be prepared to pay.
Most medium format cameras will not have in-built light meters or auto focus and are generally fully manual. Many use leaf shutters, where the shutter is built into the lens instead of the body, leaf shutters have a maximum speed of 1/500". Some later models may use electronic shutters and will therefore require batteries.
Fortunately, with the resurgence of film photography, there are any number of places to find reviews and how-to guides for just about every camera.
Medium format will almost certainly cost more to get started with, and to use, than 35mm film. The cameras are usually bigger, heavier, and more unwieldy but, it's totally worth it in my opinion. Once you start printing or scanning your negatives you will start to really appreciate the benefits.
Here are the most common images sizes from 120 film and examples of some cameras that produce those sizes.
Note - the actual image size will be around 4mm smaller in each dimension than the stated size, for example a 6x6cm image will actually be 56 x 56mm:
6 x 4.5cm - 15 shots per roll
Mamiya M645, Pentax 645, Contax 645, Fujifilm GA645, Bronica RF645
6 x 6cm - 12 shots per roll
Mamiya 6, most Hasselblad cameras, Bronica SQ, and TLR cameras, including Mamiya, Rollieflex, Minolta, Yashica, Seagull
6 x 7cm - 10 shots per roll
Mamiya RB67/RZ67, Mamiya 7, Pentax 6x7/67, Plaubel Makina 67, Fujifilm GW670, Bronica GS-1
6 x 8cm - 9 shots per roll
6 x 9cm - 8 shots per roll
Fujifilm GW690, Mamiya Super 23, many "folding" vintage cameras from makers like Zeiss, Agfa, Voigtlander, Kodak, Konica, etc
6 x 12cm - 6 shots per roll
Horseman SW-612, Kraken 612, Linhof Technorama 612
6 x 17cm - 4 shots per roll
Tomiyama 6x17 Art, Fujifilm GX617, Linhof Technorama 6x17, Horseman SW-617 Pro, Shen Hao TFC617-A
6 x 24cm - 3 shots per roll
Fotoman 624, Tomiyama Art Panorama 240, Gaoersi 624 Professional