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Global & Rolling Shutters

September 13, 2013

A camera’s shutter determines how and when light gets recorded during an exposure. In this article, we’ll explore the various shutter mechanisms that have been implemented, ranging from early film to recent digital cameras.

BACKGROUND

With film, the shutter is simply a spinning disc between the rear of a lens and the film strip. It has an opening that lets in light once per revolution for each exposure. When the light is obstructed, the film advances to the next frame and the process repeats. For these reasons it’s also often referred to as a mechanical or a rotary shutter.

Mechanical Shutter
Sensor-Based Shutter
Note: Actual rotary shutters are often more sophisticated than the wedge-shaped example shown above, but the underlying principle remains the same. Also, for the purposes of this article, the term “photosite” can be used interchangeably with “pixel,” but strictly speaking, the two refer to different concepts.

With most digital cameras, the shutter is controlled by the sensor itself. Photosites are read row by row in rapid succession, then reset between exposures, then read again for the next exposure. This simplifies things by reducing the number of moving parts, and permits full shutter angle control since film no longer has to advance between frames.

ROLLING SHUTTER

Although both technologies record light for the necessary duration, not every portion of the image starts and stops receiving light at the same time. The process is commonly referred to as a “rolling shutter” since exposures typically move as a wave from one side of the image to the other. Both film and digital work this way. With a rotary shutter, this happens as each edge of the disc’s opening sweeps across the sensor. With a sensor-based shutter, this happens as rows of photosites are initiated and terminated in rapid succession:

A rolling shutter typically goes unnoticed, and has been used to record virtually every film over the last century. In most cases, the initiation and termination phases happen so quickly that most of the time is spent with the frame either fully exposed or obstructed. However, with slower sensors, the initiation and termination phases may begin to occupy a greater fraction of each exposure, and can even happen simultaneously as a stripe that sweeps across the sensor:

Exposure Moving as a Stripe Across the Sensor

In that case, fast-moving subjects can appear angled or sheared, and rapid camera movements are more likely to appear as a wobble. Effects are most pronounced with whip pans, fast subjects, high shutter speeds or run and gun type shots. Strobes can also partially illuminate the frame if they go off when the full sensor area isn’t collecting light.

Horizontal Motion
(object moving fast relative to sensor speed)
Recorded Frame
(position shifts as shutter sweeps downward)

Despite its origins with film, the term rolling shutter is more commonly associated with digital, primarily because its effects vary more depending on the camera. Stills cameras with a video mode, compact cameras and digital camcorders all typically exhibit the strongest effect.

HYBRID SHUTTER

Some cameras utilize both a mechanical and a sensor-based shutter. The rotary shutter is typically placed on a standard digital sensor in order to compensate for a slower sensor read speed. Although this doesn’t change anything about the underlying sensor, it can reduce the impact of a rolling shutter for certain shutter speeds.

It works by using the rotary shutter to obstruct light when the sensor would otherwise be initiating or completing each exposure. If the rotary shutter is faster than the sensor itself, then a greater fraction of each exposure is spent with all photosites collecting light simultaneously:

A hybrid shutter is often also referred to as a “global shutter,” but strictly speaking, these are still standard rotary shutters that have been adapted for digital use. Rolling shutter artifacts are still possible—especially at higher frame rates and shutter speeds—since the shutter still sweeps across the sensor. Furthermore, this approach is only beneficial when all photosites are able to collect light simultaneously.

Rotary Shutter Sweeping Across the Sensor

Adding a mechanical shutter to a digital sensor therefore isn’t necessarily an improvement. These can also potentially add vibrations and noise, and are an additional moving part that can fail. A true global shutter therefore requires an entirely different technology . . .

GLOBAL SHUTTER

A global shutter controls incoming light to all photosites simultaneously. At any given point in time, all photosites are therefore either equally closed or equally open. A global shutter can work either by abruptly exposing and then obstructing all photosites at once, in which case it can be thought of as a “hard shutter,” or by doing this more gradually as a “soft shutter.” Since they have no moving parts, these are sometimes also referred to as an “electronic shutter.”

Unless specified otherwise, the abrupt on/off of a hard global shutter is often what people are referring to when they speak of a global shutter. Soft global shutters are a more recent development that can also address temporal aliasing and other motion artifacts.

A global shutter is typically considered the most accurate representation of motion, and effectively addresses all of the potential rolling shutter artifacts. Since everything within each frame happens simultaneously, rapid events such as gunshots and explosions are depicted as we see them:

Note: The cloud above is not smoke, but is actually a very high speed pressure wave created by the bullet breaking the sound barrier. This type of shot would have otherwise been impossible with a traditional rotary shutter. Example without rolling shutter artifacts actually utilizes a soft global shutter; more on those here.

Strobes also appear uniform and unbroken, even compared to cameras with an otherwise fast rolling shutter:

Rolling Shutter
Global Shutter

However, despite these advantages, global shutters aren’t always the best fit. Having a subtle rolling shutter effect can be important for making footage appear more film-like, for example. Some global shutter designs can also make sensor photosites less efficient, which can in turn compromise image noise.

A global shutter also isn’t a substitute for proper technique or exposure settings. A common misconception is that a global shutter can fix flickering in artificial light, but neither shutter type fully addresses this problem. Two ways around this type of flickering are to use either continuous lighting, or a safe shutter speed and frame rate combination.

COMPLICATIONS & DISCUSSION

Modern shutter mechanisms are becoming increasingly diverse. They can be mechanical using a rotary shutter, sensor-based using the native read-out timing, or even a hybrid where both mechanisms are utilized. More recently, they can also be global using an electronic shutter that controls light uniformly. The key is being able to identify when each type is being used, and how these might influence the resulting imagery.

Ultimately, each shutter type is just a different way to record motion, similar to how different shutter angles have various applications. The optimal solution is having a shutter than can adapt to the various subject and style possibilities. For the vast majority of scenes, a fast rolling shutter is well-suited since this has been the standard for over a century. However, with select specialized applications, sometimes a global shutter can be helpful. In that case, the RED MOTION MOUNT is one possible solution.

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