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