Filmmaking is ‘simply’ the art of capturing motion pictures. In this post, we will explore how shutter angle controls the amount of motion captured within each frame and how the 180 degree shutter rule can be used to create cinematic video. While we use the term ‘rule’ it is by no means mandatory for ‘correct’ cinematic filmmaking. It is, however, a tried and true method to produce aesthetically pleasing video and brings us back to the days of film. You can see this in our EVOLVE 2 Camera.
What is Cinematic Video?
Cinematic video closely mimics human vision. It is more natural-looking and familiar. The familiarity is what draws in audiences and makes video more visually appealing.
To create the ‘proper’ amount of motion blur (the amount of motion captured) that resembles real life, filmmakers adjust shutter angle. Shutter angle is simply the shutter speed relative to frame rate and controls motion blur.
What is Shutter Speed?
Shutter speed is the rate in which the shutter opens and closes during each frame. It is measured in fractions of a second and determines how much light hits the sensor. The smaller the fraction the faster the shutter speed. Video shot using a shutter speed of 1/800 is faster than a shutter speed of 1/125.
The faster shutter speed lets less light into the camera and therefore produces darker scenes and less motion blur as compared to a slower shutter speed, which results in a lighter video and comparatively more motion blur.
What is Frame Rate?
Frame rate is the number of frames captured per second and/or projected on to a screen. Your frame rate selection will depend upon the desired look for the project.
Almost all cinematic video is shot in 24fps, which produces buttery smooth imagery. 30fps is often used for broadcast TV, while faster frame rates (60fps and 120fps) are primarily used for slow motion and nature scenes.
Back in the Film Days 180 Degree Shutter Rule Expanded
Ready for a quick history lesson?
The 180 degree shutter rule dates back to the days of film when cameras used a rotary shutter.
The shutter angle determines exactly how much light hits the sensor before manually advancing to the next frame. Because of the time required to manually advance to the next frame, the maximum shutter angle in film cameras is 180 degrees.
At 180 degrees, the resultant image appears more natural, flowing and close to what the human eye sees.
Shutter angles less than 180 degrees decreases the shutter speed, limits the amount of light that hits the sensor and limits the amount of motion captured within each frame.
The resultant video at smaller shutter angles is therefore choppy and appears less natural.
The 180 Degree Shutter Rule in Digital Cinematography
Digital cameras do not have a physical shutter but the 180 degree shutter rule is still used and refers to the relationship between shutter speed and frame rate where the shutter speed is set to twice the frame rate.
To put it into a simple equation, the 180 degree shutter rule is 1/2x Frame Rate.
Following the 180 degree shutter standard, video shot at 24fps should be set to 1/48th of a second and 30fps should be set to 1/60th of a second to produce the most cinematic-looking video. At these settings the resultant video is buttery smooth and closely resembles how the human eye sees.
Of course, as with any ‘rule’, the 180 degree shutter standard can be broken. Depending upon the look the filmmaker is trying to achieve, it may be better to shoot with a higher shutter speed.
Fast action, TV or documentary-style projects may warrant higher shutter speeds to achieve a choppy or stutter look. Often, creators will purposefully shoot at higher frame rates to produce a stuttered effect for flashback scenes or when shooting live action sports.
Overexposure and How to Compensate
A common pitfall when shooting with a shutter speed of two times the frame rate is overexposure. But fear not, there are simple solutions to correct exposure without having to abandon the 180 degree shutter rule.
One way to correct exposure is to adjust the ISO setting. For full creative control over exposure filmmakers can manually adjust the ISO setting.
Doubling the ISO is equivalent to halving the shutter speed. For example, taking your ISO from 400 to 800 is equivalent to taking shutter speed from 1/60 to 1/30. In scenes that are too bright lower the ISO sensitivity.
In scenes that are too dark you’ll want to increase ISO. Because the shutter speed is still two times the frame rate the motion within each frame does not change, only exposure.
If you reach the ISO limit and the scene is still overexposed, you can also use a neutral density filter. ND filters come in different strengths and work to reduce the amount of light hitting the camera’s sensor.
Using these fairly inexpensive accessories allows the film maker to achieve proper exposure while using slower shutter speeds, extended exposures while using a wider aperture and controlling depth of focus.
To Follow the Rule or Not?
As previously stated, while the 180 degree shutter rule is most commonly used to create cinematic video, it isn’t a mandatory requirement.
Understanding its film camera origins and how the relationship of shutter speed and frame rate effects video aesthetics will empower you to unleash your creative license and ultimate control of your craft.
Use the equation of 1/2x frame rate as a baseline to create natural motion blur and adjust accordingly.
