There are three things that are certain in a digital filmmaker¡¯s life, in no particular order: late nights, taxes, and CMOS imaging sensors. Yeah, that list is a bit of an exaggeration on multiple fronts¡ªit reads more like my personal experiences until this point. Regardless, CMOS imaging sensors are really an integral part of the digital filmmaker¡¯s life, mostly because of the recent shift from CCD technology to CMOS technology in image sensor manufacturing. While in many aspects, CMOS imaging sensors brought about positive improvement over CCD, one apparent side effect of this shift is the presence of ¡°rolling shutter¡± artifacts.
Let¡¯s get this out of the way quickly. Rolling shutter is not inherent to CMOS sensors. While it may seem like all CMOS sensors have rolling shutter, that is certainly not the case. The Sony PMW-F55, Blackmagic Design Production Camera 4K, URSA 4K, and URSA Mini 4K, as well as the AJA CION, all use CMOS sensors with global shutter circuitry.
Many, but not all, CMOS sensors use rolling shutter.
What is rolling shutter? For those who are not familiar, ¡°rolling shutter¡± is a technical term referring to the way the image sensor scans the image. If the sensor employs a rolling shutter, this means the image is scanned sequentially, from one side of the sensor (usually the top) to the other, line by line. Many CMOS sensors use rolling shutters. In contrast to that, ¡°global shutter¡± is the technical term referring to sensors that scan the entire area of the image simultaneously. The vast majority of CCD sensors employ global shutter scanning. The overwhelming prevalence of CMOS sensors in modern cameras explains the increasing appearance of rolling shutter artifacts in videos and films made recently.
How a rolling shutter scans the image, from top to bottom of frame
OK, so your camera has a rolling shutter CMOS¡ªwho cares? Well, it¡¯s useful to know the limitations imposed by a rolling shutter through the artifacts that it can bring about in your captured images. Rolling shutter artifacts generally manifest themselves in scenes with lots of motion, be it through camera movement, objects moving quickly through the scene and, in the worst case, with unstabilized handheld camera movements. The most common manifestations that I¡¯ve come across involve scenes with vehicles moving quickly across the frame, and quickly spinning objects. Vehicles that drive through your frame can be rendered with slanted lines where they should be straight. For example, a rectangular city bus will be rendered more akin to a parallelogram. Spinning objects in the frame are a little more difficult to describe; they bend back on each other, creating strange arcing patterns. You may notice this in drone camera footage if airplane propellers are in the frame. Unstabilized handheld footage, especially with longer focal lengths, will take on an appearance akin to gelatin, in which the image bounces around in a warping fashion with the movement of the camera. As you may have guessed, the faster the movement of the subject in question or the camera, the more exaggerated the artifacts become.
Effects of rolling shutter; note buildings skewed in one direction
To make matters more interesting, not all rolling shutters are created equal. Some cameras exhibit rolling shutter artifacts more than others. Generally speaking, as the sensor gets larger and higher in resolution, the greater potential there is for rolling shutter artifacts; because the larger the sensor area, the longer the readout of that sensor will take. For example, the Canon 5D Mark II (now discontinued, but not forgotten) with its full-frame sensor was notorious for its ability to skew straight lines in motion, and to render unstabilized handheld footage in this gelatin-like fashion on occasion. On the opposite end of the rolling shutter spectrum is the ARRI Alexa/Alexa Mini/Amira, which all use the same ALEV III CMOS sensor block, but you would be hard-pressed to find rolling shutter artifacts in all but the most strenuous of situations. Some filmmakers have even remarked that cameras with faster rolling shutters have a more filmic aesthetic than cameras that employ global shutters.
On that note, why have a rolling shutter at all? Implementing it, in most cases, seems to only bring doom and gloom to your images over the older global shutter design of CCDs, right? Not so fast. Like basically everything that exists in this world through manufacture, the name of the game is compromise. Generally speaking, if you were to compare two equivalent sensors strictly on performance, one with a global shutter and the other with a rolling shutter, you will likely see that the rolling-shutter sensor will have less noise and a wider dynamic range while generating less heat. Now, a global-shutter sensor can be developed to compensate for those aspects, but the costs of developing and manufacturing a sensor with those capabilities will be dramatically higher. In essence, rolling shutter sensors provide good performance metrics while keeping costs down.
And how to deal with the artifacting? Preparedness is the key. If you understand the limitations of your camera, artifacts can be avoided by not shooting subjects that you know will cause image issues, or in some cases, camera settings can help alleviate certain artifacting. With rotating objects, a slower shutter speed will blur the rotation enough to obviate the rolling shutter effects. Using stabilized lenses, or other types of camera stabilizers like a Steadicam or a M¨VI can also help stave off the gelatin-cam. Even with all these precautions, you may still find yourself with artifacts in your images. On occasion, you may be able to remove subtle instances in post production, but in most cases, it¡¯s nearly impossible to completely remove rolling-shutter artifacts post capture.
Using a Steadicam support to stabilize camera
So, is it worth it to you? Some extra performance metrics on one side versus reduced artifacting on the other? For most, it comes down to cost. As camera prices go up, their sensor performance usually goes with it, reducing artifacts while keeping image quality high. But even when cost-cutting is necessary, it¡¯s good to know that you can get a high-performance camera that can make amazing images, especially when you have a firm knowledge of its limitations.
