Shutter Speed vs Shutter Angle

Posted by Brian Paris on Mar 11, 2009 in Blog |

There is something special to the picture that you see in films. There is a blur to the motion that you don’t really see when you watch video. Some of this is explained by the lower frame rate, but the Shutter Angle has a lot to do with it also.

With film cameras there is a rotating shutter that lets light in during part of the rotation and blocks it out for the rest. If a circle is divided up into arcs, the Shutter angle is the part of the circle that lets the light in. For example if the shutter angle is 90 then only one forth of the circle is letting in light. If the angle is 180 then it is letting in light half of the time. This is the normal set up for cameras and anything wider open or smaller can be used for different effects. The wider open angles create more motion blur, while the tighter angles make for sharper motion for high speed stuff.

Most people coming from still photography are used to setting the exposure with shutter speeds. In photography the shutter is usually a gate that opens up for a specific time and then closes. Since it is one picture at a time, the shutter speed can be any length of time that you want, from thousandths of a second to many minutes. In cinema that isn’t possible since the camera is shooting at a specific frame rate (normally 24 frames per second). If the shutter angle is 360 degrees then the light is hitting the film for the entire 1/24th of a second. If the shutter angle is 180 then it is only half that time – so it would be 1/48th of a second.

Both shutter speed and shutter angle control the time that an image is being exposed; they just get there with different ways. This is one of the primary adjustments that you can make when setting proper exposure.

Digital cameras use an electronic shutter to achieve their shutter speeds. The camera just records for a certain amount of time for each frame. The exposure come out to the same result, but there are subtle differences when it comes to the look of the finished product. The one good thing about electronic shutters is that they can be set up to compensate for changing frame rates when you are ramping the frame rate in a shot. For example, this allows you to go from a normal 24 fps to a slow mo of 60 fps without the change in exposure that you would expect if there was no compensation.

Wikipedia has all of the details of shutter angle and shutter speed.

There is a good recap of a REDuser thread on the ProVideo Coalition Website that talks about how to convert from shutter angle to shutter speed.

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