The rolling shutter effect explained

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Image courtesy: gui577b

In a nutshell

  • Rolling shutter is a more extreme distortion of the image caused by the shutter of certain cameras
  • Some cameras, such as those with CMOS sensors, do not capture the entire frame of their image in a single moment
  • The rolling shutter effect can occur in any camera lacking what is known as a global shutter

The rolling shutter is both a popular and complex visual effect. Most people don’t even realize what it is or why it happens. But if you’ve ever shot footage on your phone or captured an image on your video camera, and it’s distorted or wavy, then it’s safe to say you, too, have come across the phenomenon. To some video shoots, it can be a pain, but for some, it is a way to add some pretty cool special effects to their footage.

The rolling shutter: a closer look

The rolling shutter is a type of camera shutter. A shutter is a device that controls the amount of light to enter the camera. In the case of rolling shutters, it doesn’t expose the entire sensor to the light all at once. They expose different parts of the sensor at different points in time.

With many digital phones and cameras, the process of capturing an image or footage isn’t concurrent across the entire field of view. Instead, the sensor scans the image — either from top to bottom or from side to side. This is known as a rolling shutter.

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Rolling shutters are universal and are found in many video cameras, ranging from DSLRs to smartphones. Typically, they’re found in CMOS sensors.

About the rolling shutter effect

The effect is a mode of wavy distortion that happens when using a camera with a rolling shutter. It mostly occurs when the motion of a subject is moving too fast for the camera’s sensor to capture properly.

Say you take out your camera while on a plane and film its propellers. The blades will end up looking warped and eventually appear to bend back into their original shape as they rotate. This is the rolling shutter effect at work. The effect can happen any time you’re filming something fast.

The effect can also occur in still or static shots when the camera is in rapid motion. If you’re in a moving car and snap a shot of someone walking along the street, you will likely see some warping in the image.

Why does rolling shutter happen?

The effect tends to arise in smartphones and cameras that lack what is known as a total shutter or global shutter. When a camera has a global shutter, its shutter exposes the entire sensor all at once. A global shutter sidesteps the rolling shutter distortion effect because it captures the full visual or image at once.

That’s far from the case with cameras containing rolling shutters. These cameras contain active-pixel sensors (CMOS) that normally engage rolling shutters in order to capture images. Cameras with rolling shutters have CMOS sensors that are not fully exposed simultaneously. Instead, the cameras have a “rolling” quality to their exposures. This occurs at the maximum frame rate of the specific sensor. For example, if the sensor operates at 30 Hz or 30 fps, rolling shutter occurs over 1/30 of a second.

When is it most noticeable?

CMOS sensors that use the rolling shutter process are more inclined to send the image at a quicker rate. This also empties the sensor for the next shot. Sensors need to take 24, 30 or 60 frames a second. This is why almost all iPhone, DSLR and smartphone sensors use rolling shutters. This is also when the effect of a rolling shutter will be most noticeable, especially when recording or photographing fast-moving objects.

When the shutter rolls for about 1/30 of a second, movement is barely noticeable. However, the effect becomes more noticeable when the object is moving quite fast, like helicopter blades or a rotating car wheel. The effect also becomes more noticeable under certain lighting conditions. Many indoor lights don’t illuminate nonstop. In fact, most household lights running on electric power will go on and off because the power frequency is not uniform. Electric power in an average home is an alternating current, running at 60 Hz in the United States and usually 50 Hz in other places. This can also show up in rolling shutters as patterns of light and dark across your footage or image.

Rolling shutter vs. global shutter

The difference between a rolling shutter and a global shutter is rolling shutters use CMOS sensors, and global shutters uses CCD sensors. There are several reasons why camera manufacturers choose one sensor over the other, such as processing speed, cost, power, etc., but CMOS sensors using the rolling shutter system deliver faster images. Total shutter avoids the rolling shutter distortion effect by capturing the entire image at once. However, global shutter cameras using CCD technology have become outdated in today’s market. Mostly all global shutter sensors on the market are now CMOS sensors used more for high-end broadcasting technology. That doesn’t make them any less expensive than cameras using rolling shutter tech. In fact, one of the biggest differences between the two is cost.

If your camera has rolling shutter control, its benefits lie within its wide dynamic range that reduces the chance of a blowout between the sun and the edge of buildings. Where the global shutter varies is that all pixels within the frame of the lens are captured at the exact same moment. The sensor in the camera is exposed to light in one single moment of time. Unlike rolling shutters, there is no spatial or temporal aliasing, no blurring and no rolling or scanning. The sensor captures the whole image in one moment of time.

The accuracy of footage and images generated with CCD sensors and global shutters combine a link to give the global shutter an advantage over rolling shutters. That advantage is why global shutter is used in high-end filming and photography, such as shooting movies or sporting events and even short-form videos. Even traffic cameras utilize global shutters to capture license plates on vehicles speeding or running red lights. Outside of the different functions rolling shutter and global shutter may be used for, their greatest difference lies within the business and consumer price tag.

Also, unlike global shutter, rolling shutter creates some surreal and graphic effects that can be shared online for fun or content creation videos.

See for yourself

While the rolling shutter effect can be an unattractive and unwanted anomaly for some, it can also be an inexpensive special effects tool for others. Check out some really awesome tricks used to create the rolling shutter effect by going on YouTube. Once you see the rolling shutter effect in action, you will get a general idea of how it’s being applied creatively. The key is getting to know your equipment and becoming a master over the images and footage you create.

Featured image courtesy: gui577b on YouTube