A half-frame camera uses an ordinary film cartridge but fits two images in the place of one. For regular cameras using 35mm film the image format is 36x24mm. Half-frame cameras make exposures 18x24mm, effectively two portrait shots for every full-frame landscape, fitting twice as many pictures onto the film; 72 exposures on a 36-exposure roll, 48 on a 24-exposure roll. The viewfinders are therefore portrait orientation as opposed to the landscape orientation of a full-frame camera.

The half-frame format allowed manufacturers to build more compact cameras and offered consumers an alternative to other less common subminiature formats. Getting two shots for the price of one also provided consumers with an economical option when the costs of using color film were still very high. The Olympus-Pen was the first camera in Japan to offer the half-frame format in September 1959.