Smartphone filmmaking: The Rolling Shutter Problem
Ever shot video from a car or train window? Did you find everything in the image slanting across the screen. Did supposedly straight objects such as building now look like they’re made of long grass and there’s strong wind blowing?
Welcome to the phenomenon created by the “rolling shutter” in your smartphone camera. In fact, it’s not just smartphones but other digital cameras too.
Not only can this effect look ugly or distracting, but it can also be a nightmare for any CGI (computer generated image) work that needs to be done to the image in post. If you want to track motion and/or replace parts of an image, it doesn’t help at all if the image is bending and distorting all over the pace.
What causes it?
Cameras that shoot using good old physical film don’t have this rolling shutter problem. That’s because the whole frame is exposed to light at the same time.
However, smartphones and some other digital cameras have CMOS sensors and these sensors scan the exposed frame one line at a time, from top to bottom. Therefore, there’s a very slight time difference between when the top line is captured and the lines beneath.
Normally, you won’t notice anything. But when something is move fast either right to left or left to right across the frame, things start to look a bit strange.
That’s because the top of this object was filmed slightly before the bottom. The length of time between each line of the image exposed graduates as you move down the object. You’re seeing a kind of visual representation of time travel within one frame as the top of the frame is “older” than the bottom.
Mind bending?
Well, it’s certainly image bending. Anyone working with digital video will have encountered this problem at some point. And the culprit is what’s known as a CMOS imaging sensor.
As it turns out, CMOS sensors are the ones in most smartphones. On a positive note, smartphone camera sensors are small. And larger sensors with higher in resolution have greater potential for rolling shutter issues.
The alternative to a rolling shutter is something called a “global shutter”. As mentioned above, this exposes the frame in one go and negates any slanting objects or wobbly-looking fan blades.
So why have these rolling shutters if they cause problems? Well, rolling shutters have advantages over global shutters. Generally speaking, the rolling-shutter sensor causes less noise and allows a wider dynamic range, whilst it generates less heat.
It would be possible to create a global shutter without these issues, but it would be expensive. So a rolling shutter is an affordable way of improving image quality while keeping the cost down.
How to deal with it
You can avoid or reduce rolling shutter artifacts (or leaning buildings and other objects) by avoiding filming from a moving vehicle. You can also avoid panning left and right too fast.
Handheld camerawork, where you want that urgency in the shot, tends to cause the most problems. However, with your shot movement smoothed out using a stablizer or gimbal, there shouldn’t be a problem.
So, the obvious solution is to move the camera less. Mount it to a tripod or gimbal.
If you are able, use the slowest shutter speed possible. The motion blur created will help to reduce the rolling shutter effect. You can change shutter speed using a camera app like FiLMiC Pro.
Fix it with software
iMovie on macOS (but not iOS devices)has a stabilization effect. It’s not specifically for rolling shutter repair, but Apple claims the app can be used to reduce some unwanted effects of a rolling shutter.
On the other hand, professional editing software such as Final Cut Pro X and Adobe Premiere has dedicated rolling shutter repair tools able to fix or reduce the rolling shutter effects in an image.
This Adobe help page describes how to use Premiere Pro to repair rolling shutter issues.
Here’s Philip Bloom correcting some rolling shutter effect using iMovie 11 back in 2011:
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