While 35 mm refers to the film, 55 mm most likely refers to the size of a lens or filter.
When dealing with film cameras, a 35 millimeter (mm) camera is a standard starting point for beginners in photography. While most people can identify a 35 mm camera, not everyone understands where that measurement originates. It refers to the size of the film such a camera requires, although other camera measurements in millimeters can make this confusing. It's important to know about 35 mm cameras with regard to film, lenses and filters.
35 mm Cameras
The term "35 mm camera" describes a camera that uses 35 mm film. This measurement comes from the width of the roll of film, including the sprocket holes. These cameras include point-and-shoots, which are fully automatic with no exposure controls. An SLR (single-lens reflex) is another type of 35 mm camera. It usually offers full control over aperture, shutter speed and focus, and features TTL (through-the-lens) viewing to create accurate compositions. A rangefinder is an example of a 35 mm camera that offers manual controls but has a viewfinder instead of letting you see exactly what your lens does. Rangefinders are often lighter and quieter than SLRs.
Medium-Format Cameras
The next film size larger than 35 mm is referred to simply as "120 film," or its double-length "220 film," both used with medium-format cameras. The size of the negative depends on the type of camera you use. Formats include a 645 (6 centimeters [cm] by 4.5 cm), 6x6 (6 cm by 6 cm), 6x7 (6 cm by 7 cm) and 6x9 (6 cm by 9 cm). Although 120 and 220 film has a width of just over 55 mm, It is more likely that the term "55 mm camera" would be used for the focal length of a lens or the diameter of a filter.
Lens Focal Length
The focal length of a lens determines whether a scene looks exactly as your eye sees it, or whether it appears magnified or at a wider angle. Zoom lenses are common on 35 mm cameras; they may offer a variety of focal lengths from wide-angle to long, such as 22 mm to 80 mm lenses. Prime lenses have a fixed focal length, such as 50 mm. A 55 mm lens can easily be a prime lens offering, or represent the normal focal length of a zoom lens, such as 55 mm to 200 mm.
Lens Filter Diameter
You may also see 55 mm listed as the filter diameter of your lens. Filters are commonly used for protection of the lens, or for color and contrast effects. Filter diameters can range from approximately 50 mm to 80 mm, depending on the make and model of your lens. This measurement is either printed on the back of the lens cap or on the glass next to the lens. This number precedes or follows the symbol of a circle with a line through it, which helps differentiate it from focal length. A common measurement for a lens filter is 55 mm.
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