Cameras

CMOS Is Winning the Camera Sensor Battle, and Here's Why

Dec 29, 2011 6:00 PM
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Camera-equipped cell phones and digital cameras are packing in abundant video-capture options and in-camera extras, thanks to image sensors that consume less power while delivering high performance and advanced features.

The sensor technology, called CMOS, is increasingly being used in today's cameras, allowing users to shoot 1080p video and apply complex imaging effects with ease. Traditionally, CCD sensors have been thought to produce better-looking images with less visual noise and distortion, but they draw more power and provide slower data-throughput speed.

The shift to CMOS helps to explain how cameras have been able to evolve so dramatically in recent years, attracting the growing number of consumers who want high performance on the go.

The Rise of CMOS

Pocketable cameras now offer optical-zoom ranges approaching an impossible-sounding 20X. Similarly small interchangeable-lens cameras provide DSLR-like performance without all the heft. If you look around on the street, you'll see more people snapping photos with phones. Imaging devices are becoming smaller, more powerful, and more versatile--simultaneously.

Tucked inside all those cameras, CMOS (complementary metal-oxide semiconductor) sensors are being used as the building blocks for that versatility.

"Sensor speed by itself may not be something that people can see the great value in, but sensor speed together with processor power allows the CMOS sensor to realize features that you likely couldn't do with CCD," says Mark Weir, senior manager of technology for Sony Electronics. "When you can capture at very high rates of speed, all of a sudden, capturing a 'picture' is really about capturing many pictures. It becomes a question of 'Now that I can capture many images every time I want to take just one, how can I enhance what I've got?'"

The Shortcomings of CCD

The fact that CMOS sensors are capable of performing some of the heavy lifting themselves--image-processing tasks such as analog-to-digital conversion and noise reduction--gives the sensor technology an edge over CCD (charge-coupled device) sensors when it comes to speed. With CCD, key processes such as analog-to-digital conversion take place outside of the sensor.

"You can't get the data off the [CCD sensor] quickly enough, because there is a limit to the number of readout channels," explains Canon's Chuck Westfall, technical advisor for the company's Professional Engineering and Solutions Division. "With a CCD sensor, you're usually limited to two readout channels, and in the case of CMOS, it's basically up to the designer as to how many channels they want to put on there. We've got a 16-channel readout on the [CMOS-based Canon EOS] 1D X, for example. We have an 8-channel readout on some of our other cameras. And even in the compact cameras, they don't specify, but I'm pretty sure it's way more than two."

Today, if your camera has a very fast continuous-shooting mode, it has a CMOS sensor. If your camera uses exposure bracketing for low-light shooting, or if it captures superslow-motion video and compiles high dynamic range shots, you own a camera with a CMOS sensor.

And in the imaging business, CMOS's recent widespread adoption is somewhat of an upset victory.

Nimble Is Better

Not too long ago, these flexible sensors were largely considered second-rate components next to their CCD counterparts. And CCD still has advantages related to shutter mechanics during video capture.

The "rolling-shutter effect" is a frequent knock on CMOS sensors, and it's a concern when you're shooting video, trying to capture a fast-moving subject, or panning the camera while shooting. Pixel by pixel, CMOS sensors scan what is in front of the lens, almost as if they were reading a book; each row of pixels captures what it "sees," line by line, in rapid succession.

As a result, each pixel isn’t capturing the same thing at the same time, and sometimes you'll see odd artifacts in video and images captured with a CMOS sensor: tilted vertical lines, distorted moving objects, and horizontal bands of light. CCD sensors, on the other hand, capture the entire frame at the same time, using what's called a global shutter.

Although companies have developed CMOS sensors that employ a global shutter, it may be quite a while until the technology is found in consumer-level cameras.

"There is such a thing, but it tends to be in more of an experimental stage at this time," says Westfall. "There's a company called Dalsa that manufactures a global-shutter CMOS sensor, but it's not for general consumer cameras. It's only for industrial-type cameras, and it tends to be far, far more expensive."

CCD Still Has Advantages

These days, however, it's hard to find a current-generation camera that carries a CCD sensor.

When you do find one, it's usually at the very high end of the premium point-and-shoot market--Canon's PowerShot G12, Nikon's Coolpix P7100, Olympus's XZ-1, and Panasonic's Lumix LX5, for example--where the potential user is primarily interested in still-image quality.

"The [Coolpix] P7100 is all about giving the user a more 'pro-like' experience, so the need for a super-high resolution [CMOS] sensor was not as advantageous in this particular design," says Steve Heiner, senior technical manager for Nikon. "Other aspects of performance are more important, such as a global image sensor shutter for high-quality 720p movie files with no rolling-shutter effect."

"The faster the scanning and readout capabilities of the particular CMOS sensor used, the less rolling shutter effect [will be] seen in movies," says Heiner. "Until sensor technologies progress to a point where the scanning is as fast as a global shutter, we will see this slight difference between the two types of sensors, as the global shutter is hard to accomplish in current CMOS designs. I will say that this aspect of CMOS camera performance is getting better in nearly every generation of new products."

Next page: The CMOS Advantage