Shutter-speeds and the 180 Degree Rule

In class last week, it was discussed that to achieve a ‘filmic’ look, you must always set your shutter speed to twice that of your frame rate.  I was interested to understand why this general rule exists and the effects that can be achieved by breaking it.

The rule, as with most things in both digital video and photography, comes from the days of film. A film motion camera (and some hi-end video cameras) uses a physical shutter in the shape of a half circle that rotates over the film plane, the point of axis just outside the film plane.  As the shutter rotates then, when the half circle is outside the film plane the film is being exposed. When the half circle is inside the film plane, the film can be advanced to the next frame.  The shutter, defined as a half circle, will then give even time to exposure and film advancement.  The two halves of the shutter circle, expressed as having a shutter angle of 180 degrees, allows on revolution for each frame.  As such, the shutter is limited to allowing an exposure time half that of the frame rate: at 1fps, the shutter can only allow half a second of light with the shutter being at an angle of 180 degrees.  Applying this same maths to 24fps, your maximum shutter speed must be 48th second, or twice the frame rate.  Likewise, overcranking to 60fps will mean the exposure time should be 120th second. While cameras could increase the shutter speed by changing the angle of the shutter, a shutter angle of 180 degrees renders the best motion.

Thus, to achieve a ‘film’ look with a digital camera, it is always best to set the shutter speed at half that of the frame rate.  In doing so, motion will be optimally rendered (enough motion blur to render motion smooth without become messy) but also will deliver the audience with an experience inline with convention – motion will render the same way a professional film will.

Of course, rules were meant to be broken.  Shutter angles (or speeds in Digital motion capture) can be sped up to create a kind of staccato motion effect. This is used most famously in the opening scenes to Saving Private Ryan, where the audience becomes somewhat disoriented by the lack of motion blur despite copious camera and scene movement.  On the other hand, slow shutter speeds have been used effectively in Baraka, where under-cranking and slower shutter speeds abnormally renders motion.

Here is a video that shows the effect of fast and normal (180 degree shutter) shutter speeds.