56 x 41.5 mm image area · 10 interactive lens diagrams
6x4.5 film SLR and autofocus medium-format systems | Nominal 56 x 41.5 mm film image area with a roughly 70 mm diagonal
6x4.5, often called 645, is the most compact common roll-film medium format. The name is nominal rather than literal: the usable frame is closer to 56 x 41.5 mm, depending on the camera gate, and the long side follows the width of 120 or 220 roll film. It preserves much of the larger-negative advantage over 35 mm while giving more frames per roll than 6x6 or 6x7 and a rectangular image shape that maps naturally to many print and magazine layouts.
Optically, 645 lenses need a much larger image circle than full-frame lenses, and SLR mirror clearance often increases back-focus demands. Normal lenses sit around 75 to 80 mm rather than 50 mm, wide angles require significant coverage, and fast apertures become physically large quickly. The format's catalog entries show medium-format design priorities: coverage, even performance across a large field, restrained distortion, and manageable lens size for handheld or studio camera systems.