In conventional motion picture cameras, a film is moved from one frame to the next by a film pull down mechanism usually in the form of film pull down claws which engage the film perforations. A cooperating shutter mounted for rotation includes an opaque section over at least 180.degree. for eclipsing the film in the aperture during the pull down time.
In many pull down mechanisms, the rate of movement of the pull down claws from their initial point of engagement of the perforations to the point where they release the perforations corresponds approximately to the same time for the claws to return to again engage the perforations and release the perforations. In other words, the pull down time is about equal to the dwell time of the film in the aperture. Under these circumstances, the maximum exposure for each film frame can only take place during 180.degree. of the entire 360.degree. operating cycle made up of the pull down time plus the dwell time.
In reality, the exposure time is less than 180.degree. since it is desirable to only expose the film frame when the aperture is completely open by the shutter. Since the aperture has physical height and length dimensions, the same is only completely eclipsed after the leading edge of the shutter has left the far corner of the aperture and before the trailing edge leaves the near corner of the aperture. These corners subtend an angle which is thus subtracted from the 180.degree. during which it would be desirable to expose the aperture. If such angle subtended by the corners of the aperture is 80.degree., then there is in reality only an exposure of the film frame during 100.degree. of the total 360.degree. cycle.
By a special design of pull down claws involving levering action in cooperation with a rotating pivot, the pull down time can be made less than the dwell time thereby enabling an increase in the exposure time. Some camera manufacturers provide an arrangement wherein exposure takes place during times when the aperture is not completely open that is, corner portions of the aperture are exposed and the quality of the resulting image is sacrificed simply in order to increase the exposure time for each frame.
With the advent of different aspect ratios for film frames, it would be desirable to provide in a motion picture camera some means of changing or varying the actual length of film moved during each pull down time. By such an arrangement, the camera would be adaptable to film frames of different dimensions. Under normal conditions, for a particular frame size, there must be designed a specific camera with a specific pull down mechanism.