CMOS Is Winning the Camera Sensor Battle, and Here's Why
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
The CMOS Advantage: Beyond Image Quality
For the most part, CMOS's comparative benefits lie outside the realm of image quality altogether.
Traditionally CMOS chips have been associated with theoretically cheaper manufacturing costs, greater energy efficiency, faster data-throughput speeds, and the fact that the on-sensor circuitry handles some processing tasks before the data comes off each pixel and the sensor itself.
Of course, a camera's image processor also has a lot to do with the final image, and those are becoming more and more powerful, too.
"I would say that the processing pipeline has a lot to do with it," says Sony's Mark Weir. "It has a profound effect on the overall image quality. A lot has been written about cameras that purportedly have identical sensors--the guys at DXO, for instance, do very in-depth analysis of who's using what sensor--and they've demonstrated convincingly that two cameras using the same sensor can deliver very different results."
CMOS Advantage: Speed
One of the first showpieces for CMOS's high-speed potential was 2008's Casio Exilim Pro EX-F1, a fixed-lens, CMOS-based camera that could shoot 60 images per second at full 6-megapixel resolution, or even 1200 fps at much lower resolutions.
Since then, many major camera manufacturers have been adding high-speed still- and video-capture modes into their own CMOS-based cameras, as well as other features that harness the potential of fast capture and data-shuttling speeds.
CMOS Advantage: Autofocus and Video
"There's plenty to be said for what CMOS can do for on-sensor autofocus," says Weir. "For instance, image sensors are being called upon to do a lot more than image capture these days. With mirrorless cameras and mirrored cameras that are shooting video [with the reflex mirror flipped up], the image sensor has to do an awful lot more, and it ends up being part of the camera-driving system, not just the recording system. So frequently, large-scale image sensors in SLRs, and also small-scale imagers in point-and-shoot cameras, are being called upon for contrast AF calculation. This is an area where sensor speed and sensor readout, which can be realized with CMOS, really makes a difference. It would be very difficult, if not impossible, to do that with CCD."
Over the past few years, Sony has been at the forefront of translating CMOS's high-speed capabilities into some groundbreaking in-camera features.
Across the company’s entire camera lineup, you'll find features such as 1080p video capture at 60 frames per second, high-speed still-shooting modes, automated exposure bracketing for low-light shots and HDR, motion-controlled panorama modes, and 3D shooting with a single lens.
"Speed jumps off the page at you as 'shooting speed,'" says Weir. "In general, the multishot capture for a variety of in-camera features--Sweep Panorama, 3D Panorama, anti-motion blur, in-camera HDR, and background defocus--all of those things leverage the capture speed of the image sensor to make those features practical."
"And that crosses over to video, too. You can't reach 60 progressive frames per second with CCD, not at all. CCD has trouble with progressive scanning to begin with. So I think, certainly in terms of the throughput required by full HD video, it's an area where CMOS becomes an enabler."
CMOS Advantage: Low-Light Shooting
The benefits of CMOS now extend beyond speed, too, thanks to some developments in sensor technology that have occurred in just the past few years. In the world of compact cameras and phones, the development and widespread adoption of backside-illuminated CMOS sensors have changed the nature of what smaller-sized CMOS-based cameras can do in dark situations.
Developed by Sony and released in 2009, the backside-illuminated CMOS sensor moved the light-blocking wires--ironically, the same ones that help make CMOS sensors more efficient in data-readout speed--to the back of the chip. As a result, smaller CMOS sensors became much more effective in low-light situations, and they began to narrow the image-noise gap between CMOS and CCD. Now, backside-illuminated CMOS sensors are found in everything from high-end compact cameras to the Apple iPhone 4S.
"One of the reasons why CCD in the small form factor tended to be a little better than CMOS--I'd say roughly three or four years ago--was because more of the actual surface area was sensitive to light," says Canon's Chuck Westfall. "Now, with the back side illuminated, that's no longer the case. If there's actually no advantage to the CCD in terms of noise, at that point the CMOS advantages where you have 1080p versus 720p start to win out."
Canon's Migration to CMOS
You can see the CMOS advantages winning out just by examining Canon's current line of point-and-shoot cameras.
Over the past few years, Canon has introduced the "HS System" of CMOS-based PowerShot cameras, signaling a near-complete transition from CCD to CMOS for the company's compact-camera lineup. That includes the performance-oriented, CMOS-based PowerShot S100, which currently tops our chart of point-and-shoot cameras with the best image quality. The S100's predecessor, the S95, had a CCD sensor, and the camera industry outside of Canon took notice of the generational CCD-to-CMOS shift.
"The [PowerShot] S90 and S95--and also the G12--were some of the big holdouts on CMOS, and some of the bigger supporters of CCD," says Sony's Weir. "But I think the S100 was kind of a sea change. You can see the kinds of HS [CMOS] technology that they've been building into point-and-shoots for a couple of years now. I think that's a good example of 'having the sensor isn't enough.' It's having the processor power and sensor design to harness what the sensor can really do that's making the difference. That part of it probably took them a little longer."
Canon underwent a similar CCD-to-CMOS shift in its DSLR line quite some time ago.
"When we were talking about replacing SLR sensors with CMOS, which happened all the way back in 2000 with the EOS D30, the big story was how we were able to reduce the noise level on a [large-size] CMOS compared to a CCD," says Canon's Westfall. "It had to do with some proprietary technology that Canon developed and has since been emulated by others. It was a big step forward for the entire industry. We're much further along in the area of noise reduction in 2011 than we were in the year 2000."
"On the point-and-shoot side, there weren't any CMOS sensors for any small-format camera, be it still or video, until just a few years ago," Westfall says. "Part of the reason for that is because as you got into those small-chip cameras, the size of the individual pixels became much smaller, and the noise-reduction accomplishments that we were able to do with SLR-size [sensors] weren't happening. It took some additional technological development to be able to really get to the point where we are today."
Lytro Camera Hints at CMOS Future
Right around the corner, we'll be seeing another example of how CMOS's versatility is helping to enable in-camera innovation. Lytro's light-field camera, which lets users refocus images after taking them, uses a custom-modified CMOS sensor in its first-generation model.
"What we do is a custom package with the sensor, a package with what we call a micro-lens array," says Lytro executive chairman Charles Chi. "It fits right on top of the sensor itself, and that's what creates the light-field sensor--in addition to a lot of software and processing that comes afterwards."
"In our first product, we're using a CMOS sensor. A light-field sensor is tolerant to CMOS or CCD--in fact, it doesn't matter whatsoever. If you're like us and facing a lot of technological-innovation challenges, it's good to be riding the CMOS horse. All the benefits that you would usually get from a CMOS sensor, we benefit from as well, in terms of fast readout time. More important, I think there's so much research being done on different types of pixels for CMOS sensors. The industry is doing so much development there."