Rotary disc shutter
Encyclopedia
A rotary disc shutter is a type of shutter
Shutter
Shutter may refer to:Windows:*Window shutter, a solid window covering used to block light and winds*Plantation Shutters/Jigsaw Shutters, interior wooden louvred shutters to control heat, light and privacy...

. It is notably used in motion picture cameras.

Rotary discs are semicircular mirrors which rotate in front of the film gate
Film gate
The film gate is the rectangular opening in the front of a motion picture camera where the film is exposed to light. The film gate can be seen by removing the lens and rotating the shutter out of the way...

, and thus expose the film. As the mirror spins it reflects the image onto the ground glass
Ground glass
Ground glass is glass whose surface has been ground to produce a flat but rough finish.Ground glass surfaces have many applications, ranging from mere ornamentation on windows and table glassware to scientific uses in optics and laboratory glassware....

 so that it can be viewed by the camera operator
Camera operator
A camera operator or cameraman is a professional operator of a film or video camera. In filmmaking, the leading cameraman is usually called a cinematographer, while a cameraman in a video production may be known as a television camera operator, video camera operator, or videographer, depending on...

 part of the time. The other part of the time the mirror allows the light to pass onto the film. On simple cameras this shutter is fixed and usually semi-circular. On more advanced cameras the shape of the shutter can be adjusted. This shutter setting is referred to as the shutter angle.

Adjusting the shutter angle controls the proportion of time that the film is exposed to light during each frame interval. The angle of the shutter forms a proportion to the time that each frame of film is exposed:


The primary reason that cinematographers adjust the shutter angle is to control the amount of motion blur
Motion blur
Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single frame, either due to rapid movement or long exposure.- Photography :When a camera...

 that is recorded on each successive frame of film. A tight shutter angle will constrict motion blur. A wide shutter angle will allow it. A 180° shutter angle is considered normal.

So for instance, at 24p
24p
In video technology, 24p refers to a video format that operates at 24 frames per second frame rate with progressive scanning . Originally, 24p was used in the non-linear editing of film-originated material...

 (actually 23.976 frames per second) the "Frame Interval" value is 0.04171 sec. ( = 1 / 23.976 ). Using an "Exposure Time" of 1/50 sec. we get a "Shutter Angle" value of 173°, very close to 180° (normal motion blur effect).

This is the reason of the rule-of-thumb "Use an Exposure Time twice the value of the Frames-per-second to get normal motion blur".

Tight shutters create a stuttering stop motion animation
Stop motion
Stop motion is an animation technique to make a physically manipulated object appear to move on its own. The object is moved in small increments between individually photographed frames, creating the illusion of movement when the series of frames is played as a continuous sequence...

 look that has become popular in action and war films. In particular, tight shutters are used to capture particles flying through the air, such as dirt from an exploding mortar.
On most film cameras the shutter angle is changed by removing the lens and adjusting the shutter with a special tool. This cannot be done while the camera is operating. Some cameras such as the Arriflex 435ES
Arriflex 435
The Arriflex 435 is a movie camera product line created by Arri in 1995 to replace the Arriflex 35III line. The number reflects its position as a successor camera to the Arri III and the fact that it is designed for 35 mm film. The 435 cameras are specifically designed as MOS cameras, which means...

 can modify their shutter angle during the shot. This is sometimes referred to as an electronic shutter. An electronic shutter can compensate for the exposure change caused by a speed ramp without changing the aperture and affecting depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

.

Other types of shutter adjustments, such as an out-of-phase shutter, and a fluttering shutter are also possible. Normally the film is held steady in the gate whenever it is exposed to light. Special shutter adjustments allow the film to be exposed while the camera mechanism is actually moving the film to the next frame. In the film Saving Private Ryan
Saving Private Ryan
Saving Private Ryan is a 1998 American war film set during the invasion of Normandy in World War II. It was directed by Steven Spielberg, with a screenplay by Robert Rodat. The film is notable for the intensity of its opening 27 minutes, which depicts the Omaha Beach assault of June 6, 1944....

, cinematographer Janusz Kamiński
Janusz Kaminski
Janusz Zygmunt Kamiński is a Polish cinematographer and film director. He has photographed all of Steven Spielberg's films since 1993's Schindler's List.-Life and career:...

 used such a shutter adjustment to give his film the look of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 newsreel photography. Previously, this effect could only be achieved by purposefully mis-adjusting the timing belt of the film advance mechanism in the camera. However, several manufacturers now provide accessories to electronically control the phase relationship of the shutter and film advance mechanisms. One such device is the Timing Shift Box available for the above mentioned ARRI 435 series.

Electronic equivalent

Many video cameras also provide the ability to adjust their shutter. In most cases the camera does not have an actual mechanical shutter. Instead, this adjustment controls the amount of time that the electronic sensor collects light in order to create each successive image. Because electronic cameras do not need to mechanically advance film, it is possible for the shutter to be open for nearly the entire period of each successive frame. At 24 frames per second, it is possible to expose a frame for nearly 1/24 second, achieving longer motion blur otherwise impossible to achieve on film at sync sound
Sync sound
Sync sound refers to sound recorded at the time of the filming of movies, and has been widely used in movies since the birth of sound movies.-History:...

 speeds.

It is even possible for the shutter to be open for multiple frames, far beyond that possible with a 360° shutter angle. One example is the Sony PD170 where the shutter can be set as low as 1/3 second; accumulating light across 10 frames (at a frame rate of approximately 30 frames of 60 interlaced fields per second for the NTSC version of the camera and 25 of 50 for the PAL version)

The term electronic shutter is often used to describe the electronic process of controlling exposure time on a light sensor. The same term is also used in film cameras to refer to a mechanical rotary shutter which can adjust its shutter angle electronically while shooting.

Rotary shutters on still cameras

While the vast majority of rotary shutters are naturally used on motion-picture cameras (as well as projectors), there were some still cameras that employed this type of shutter, notably the Univex Mercury
Univex Mercury
The 1938 Univex Mercury was an unusual half frame 35 mm camera from The Universal Camera Corporation.It was the first camera with any kind of hot shoe connector, and had a distinctive profile due to a large dome protruding out the top of the main body to accommodate its 1/1000 sec rotary disc...

 and the Olympus Pen F
Olympus Pen F
The Olympus Pen F, Pen FT and Pen FV were very similar half-frame 35 mm single-lens reflex cameras with interchangeable lenses produced by Olympus of Japan between 1963-1966 , 1966-1972 and 1967-1970...

 SLR. Both of these cameras were half-frame 35mm cameras (frame size 18 x 24 mm). The rotary shutter proved to be very simple to construct, accurate and reliable in these cameras. Though, in the case of the Univex, it resulted in a very strange "hump" on top of the camera to cover the shutter disc.

See also

  • Shutter (photography)
    Shutter (photography)
    In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene...

  • Shutter speed
    Shutter speed
    In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....

  • Time-lapse
    Time-lapse
    Time-lapse photography is a cinematography technique whereby the frequency at which film frames are captured is much lower than that which will be used to play the sequence back. When replayed at normal speed, time appears to be moving faster and thus lapsing...

    – similar issues to shutter angle
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