To CCD or to CMOS, That is the Question
I was recently browsing the internet and I came across a wonderful article by Barry Green, illustrating some differences between CMOS and CCD sensors. The article laid out the differences so well I thought it would be a great topic to relay to our trusted B&H newsletter readers. For more extensive information, please check out his more comprehensive article here.
These days, it seems many prosumer cameras are making the jump from CCD to CMOS chips. While CMOS chips seem to be primarily found in consumer products, the recent release of Sony's HVR-V1U, PMW-EX1 and HVR-Z7U, shows that the adoption of CMOS technology is entering the professional market as well, even up to the ultra-high resolution RED One digital cinema camera. To be fair, it seems that diversity still reigns supreme; you'll still find a huge selection of CCD-based camcorders such as Canon's XH-A1/G1, Panasonic's AG-HVX200, and JVC's GY-HD250. I'm certain we are going to see many more cameras developed with both kinds in the future so it's probably a good idea to know something about each of them. Is there a huge difference between the two technologies? Is one better than the other? Well let's dive right in shall we!
CMOS (Complementary Metal–Oxide–Semiconductor) and CCD (Charge-Coupled Device) technology were developed roughly at the same time. By some accounts, CMOS technology pre-dates the CCD by six years; (1963 and 1969 respectively.)
Now, I'm not going to go so far as to say the sensors in the latest and greatest camcorder resembles these electronic relics, I'm merely trying to illustrate that neither technology is antiquated by any means. CMOS chips have had quite a few revisions over the years, making it very appealing, especially when trying to increase resolution. On the other end of the spectrum, CCDs have been a mainstay in the industry for decades. As of 2008, both have advantages and disadvantages over one another.
While the sensor isn't the only thing to consider when purchasing a new camera, knowing more about what each technology is capable of can really help to distinguish the right camera for you. The most important thing to remember is that getting the right tool for the job is the name of the game, especially to us professionals.
Global Shutter vs. Rolling Shutter
One difference between CCD and CMOS sensors is the way they capture each frame. A CCD uses what's called a "Global Shutter" while CMOS sensors use a "Rolling Shutter".
Global Shutter means that the entire frame is captured at the exact same time. The best way to envision this is by thinking of the sensor as a slide of film; when light passes though a slide of film, it travels through the entire frame at once. A CCD captures light much like a piece of film, all at the same time. In 3CCD camcorders, light is broken up through a prism. The red, green and blue channels are all captured separately.
On the other end, rolling shutter exposes the frame from top to bottom. Imagine a TV as it writes an image line-by-line. A CMOS sensor captures light though capturing each pixel one-by-one. Each row of pixels (from top to bottom) is captured in sequential order, creating a rolling effect, hence the name "Rolling Shutter".
The difference between the two in most real-world situations will be unnoticeable, especially in bright sunlight or evenly, well-lit rooms. Rolling Shutter, however, can be tricky in a variety of applications. For example, shooting under slow-flickering lights, such as old fluorescents, can be difficult. The effect will appear as a dark bar rolling through your footage. As the CMOS sensors gathers light pixel-by-pixel, there are changes in illumination because the light is flickering on and off.
To the human eye, the flicker of a fluorescent light is virtually imperceptible. To the camera however, some pixels end up getting more light than others. The on-again/off-again nature of the flickering light can end up becoming a very real issue.
Another situation where rolling shutter can potentially be an issue is around camera flashes. Large and very fast changes in illumination can make frames look split in half, with one-half appearing very bright and the other half appearing very dark.
During panning shots, the two sensors can also act quite differently. The CCD's global shutter will make things appear to blur as the operator performs a swish-pan, much in same way film does.
A CMOS sensor will appear to stretch or "skew" the image in either horizontal direction, making straight lines appear to bend in a diagonal fashion.
Vertical smear refers to a common CCD-related issue that most video professionals are familiar with. Since the entire image is being captured at once on a CCD sensor, shots of bright lights can often have a vertical haloing effect, causing one column of pixels to appear brighter than the rest. The problem looks something like a lens-flare.
Smearing isn't a problem with CMOS sensors, since each pixel is captured separately. Sporting events, for example, is one instance that could cause a bit of vertical smearing; if the camera should for some reason focus on those lights which illuminate a playing field at night, a CMOS sensor would have a better time handling the scenes without problems.
Power consumption can be a major factor when considering what camcorder to purchase. How many hours do you shoot per day? How much weight will all those extra batteries be? Or more importantly, how much does each battery cost? Traditionally, CMOS sensors use less power than CCDs, so for shooting documentaries, having a CMOS camera might yield significant gains in runtime per battery.
The Right Tool for the Job
The purpose of these facts isn't to try to make an argument that one technology is better than the other. As you can see, depending on what type of production you are doing, you might need a different type of camera. It's good to know what will happen in the field. At the end of the day, both sensors can provide magnificent results and staying educated will help make sure you get a great image every time.