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State of Our Industry - March 2013:

State of the Art

Rolling Shutter: What is it, and why do I care?

by Heath Firestone

History:

Not that long ago, rolling shutter wasn’t really something most people had heard of or cared about, but that was before CMOS sensors basically took over the consumer video and professional digital cinema world. I remember talking with Robin Pengelly several years ago, who at the time was the CEO of Vicon Entertainment, which owns House of Moves, the largest motion capture facility in the world, and we were discussing the challenge of using high speed, machine vision cameras for their motion capture systems, because of the difficulties caused by the rolling shutter. At that point, few people knew what a rolling shutter was, and the challenges it presented. Now, however, it affects everyone, whether they are shooting with a DSLR camera, a Red Epic camera, or their cell phone. So, what is it?

Rolling Shutter: How It Works:

In order to understand rolling shutter it is important to know how an image sensor captures and collects images. CCD’s (Charge-Coupled Device) image sensors, which previously dominated the professional video market, use what is known as a global shutter, meaning that the entire image is captured at a fixed point it time, which is consistent for the entire image. Because of this, they do not experience the problems faced by their lower cost competitor, CMOS (Complimentary Metal Oxide Semiconductor) sensors, which offer greater latitude and sensitivity, and higher frame rates, etc. In contrast, CMOS sensors generally use a rolling shutter, which actual scans the image as it captures, which can cause artifacts based on the fact that the lower portion of the screen is captured at a later time than the upper part of the frame (this is known as temporal shift). This time differential can cause all sorts of havoc, especially with fast moving shots, or when there are flashes of light. It is very difficult to correct artifacts caused by light flashes that may only illuminate the upper third of the screen. However, skew, caused by motion, especially fast panning is relatively correctable in post production using the right tools. Skew and Wobble are two of the most common artifacts, which are just a way of describing the slanting of lines, and instability of certain image elements caused by the temporal shift. Generally post processing correction is the cure, which is pretty much unavoidable for certain kinds of shots, when using cameras with a global shutter, especially with cameras like DSLR’s which are notoriously bad with regard to rolling shutter issues.

So, What Do We Do About It?:

That being said, these corrections are exactly that. They reduce the artifacting, which they do by creating a mesh warp, but they don’t generally completely remove any existence of the distortion. Since no rolling shutter is ideal, manufacturers are beginning to release CMOS sensor cameras with a global shutter. The first Digital Cinema Camera to use one of these is Sony’s F55. If you want to see an F55 in action, there is a Denver event on Tuesday, March 5th, where one will be on display. To get more info, check out CFVA’s invite courtesy of ACamera http://www.cfva.com/ebulletin/news/sony-f555-denver-premier.

Heath Firestone
Producer/Director
Firestone Studios LLC
Heath@FirestoneStudios.com

Heath is a writer/producer/director/editor, who was an early HD adopter, and who has extensive experience with visual effects, and owns the only 360 degree retro-reflective compositing stage with real-time tracking, in the world. He specializes in creating unique and complex 3D Composites, and has created shots that have never been done before. He enjoys coming up with new effects and creative concepts, that are outside of the norm, and creating visually dynamic compositions, unrestrained by traditional limitations.