This week Canon launched the latest in their lineup of of XXD cameras, the EOS 70D. It features a 20.2MP APS-C sensor, environmental sealing and 7fps continuous shooting speed with built in WiFi connectivity. What's particularly interesting about this launch though is that the headline feature of the whole launch has nothing to do with shooting photos at all. It's all about the video functionality.
There's nothing to write home about with regards to the specification of the video itself, still stuck with the same old 1080p at 24,25,30 and 720p at 50 and 60 (coming up for 4 years of this now Canon….), but they have delivered a new technology designed to offer appreciably faster AF during video shooting.
The new system is called Dual Pixel CMOS AF and from this point onwards in the article I will refer to it as DPAF for simplicity.
The Past & Present
First a little background information is useful when it comes to discussing autofocus systems. Typically DSLRs these days use two types of AF systems; Phase Detection and Contrast Detection. Phase detect AF is used for taking still images because it requires the use of the camera's mirror. Most of the light coming through your lens is bounced right up to the pentaprism and hence to the viewfinder but a small portion of the light passes through the mirror to a secondary mirror behind which then bounces it down onto a dedicated AF sensor. This AF sensor, or phase detection sensor, compares the light coming in from either side of your image. A series of microlenses (the number depends on how many AF points you have, 1 pair per AF point) measures how far out of phase the image is from one side of the lens to the other. This is a process that can be easily visualized by imagining the two converging images in the viewfinder of a rangefinder camera. The process is essentially the same, but automated at incredible speeds by the AF chip. The pair of microlenses related to to your choice of AF point can quickly assess by how much your image is out of phase and then move the lens elements to make a correction. Phase detect AF is incredibly fast and accurate but of course the problem is that it requires the mirror to be in the down position in order to reflect the light to that dedicated AF sensor.
When cameras began to implement liveview with mirror lock up and then video, a new AF system was needed and along came Contrast Detection AF for DSLRs. In reality this form of AF had been around for some time on many other video cameras but for the first time it was implemented into DSLRs. To put it bluntly, it kind of sucked to begin with and there was no way it was good enough for any kind of continuous focusing, nor were the still lenses out there designed to be used in that manner anyway. With the mirror in the up position many cameras offered two modes of focus. The first used contrast detection and the second was more accurate because it temporarily flipped the mirror back down again to employ the phase detection sensor. It was more accurate but you lost the image on the screen for a brief moment as the mirror flipped down and back up again. Contrast detection works on the simple notion that contrast in an image is highest when proper focus is achieved. The problem with this is that the lens elements actually need to move in order to generate different contrast values for the camera to evaluate and gain the data needed to estimate focus position. This is why you'll often see a camera “seeking” when using contrast detection AF. The camera is evaluating the change of contrast as it moved the lens one way and then the other. It's an iterative process and therefore it takes a lot more time which really doesn't lend itself very well to continuous focus during video shooting.
You might be wondering why your cheap Panasonic video camera has been able to focus without much trouble since the early 2000s if that too has been using contrast detection. The answer is simply down to the fact that larger sensors like APS-C or full frame 35mm sensors in today's DSLRs are simply a lot less forgiving due to the vastly shallower depth of field compared with tiny everything-in-focus video camera sensors.
The Future
Canon have begun to release lenses with the STM designation which means that they are internally designed for continuous focus during video. They are quieter and much smoother in the way that the lens elements move when powered by the AF motors. The next step in the puzzle was to come up with a new AF system that was vastly superior to the slow, iterative contrast detection method. Along comes DPAF…..
The solution was to re-design the image sensor entirely. Instead of simply having a photodiode for every pixel to collect light wavelength data as we would see in a 'normal' sensor, Canon have added a second photodiode to each pixel. This means that at a pixel level, the faster and more accurate phase detection method of focusing can be performed without the need to send the image to a totally separate AF sensor. The pixel microlenses split the image from both sides and send it to the respective photodiode for phase comparison. An added benefit of this is that the camera can use 80% of the imaging sensor for AF which is a vast improvement on the usual smaller cluster of AF points near the center of the image. In standard photographic mode with the mirror down, the two photodiodes combine again to output a single pixel and the image is transmitted to a dedicated AF sensor as usual. Canon claims that the DPAF is up to 5X faster than previous liveview shooting with EOS cameras which should instantly put it into the realms of usefulness for one man operation with a DSLR when it's not always practical to be trying to operate focus with one hand on a follow focus. The sample demo video from Canon below shows how the camera no longer 'seeks' during video AF and it looks mighty impressive, though we'll need to get our hands on one for further testing of course.
The first camera to utilize this will be the EOS 70D when it hits the store shelved in September and I'm sure we'll see this on every camera moving forwards. Most excitingly for many will be it's inclusion in the follow up to the 7D (7D MKII ?) which ought to see the light within the next six months or so since it's more than three years old at this point.
If my explanation wasn't clear enough, Canon has also made a video to explain graphically:
Canon's Sample Video
Canon produced this sample video to show the DPAF in operation. Make sure you watch through to the second half as they replay the video again but with on-screen notes displaying the lens used and the AF settings which give you a good idea how things are working.
Canon's Own Press Release on DPAF
Canon Develops New Dual Pixel CMOS AF Technology To Significantly Improve Autofocus Performance During Live View And Video Shooting
MELVILLE, N.Y., July 2, 2013 – Canon U.S.A., Inc., a leader in digital imaging solutions, today announced its parent company, Canon Inc., developed Dual Pixel CMOS AF, an innovative new autofocus technology for the EOS 70D Digital SLR camera. Dual Pixel CMOS AF, a phase-detection AF technology conducted directly on the image sensor plane, employs a CMOS sensor on which all of the effective pixels are able to perform both imaging and phase-detection AF simultaneously to achieve dramatically improved AF performance over prior EOS cameras during Live View and video shooting.
As digital SLR cameras have evolved in recent years in terms of functionality and performance, shooting styles have become more diverse. An increasing number of users no longer rely exclusively on the viewfinder when shooting, but rather view the camera's LCD monitor when using the Live View function to capture still images and when shooting video.
Canon's newly-developed Dual Pixel CMOS AF is an innovative new image-plane phase-detection AF technology that employs a CMOS sensor on which all of the effective pixels are able to perform both imaging and phase-detection AF simultaneously. Each individual pixel (the smallest structural unit capable of outputting an image signal) on the CMOS sensor incorporates two independent photodiodes (elements that transform light into electrical signals) which output signals that can be used for both imaging and the phase-detection AF. When using the EOS 70D Digital SLR camera's Live View function, the technology enables autofocusing with ease, flexibility, speed and accuracy similar to shooting through the viewfinder, enabling sharp focus to be obtained across a wide shooting area1 through phase-detection AF2 until final focus is achieved. Compared with earlier generations of Canon's image-plane phase-detection AF3, Dual Pixel CMOS AF realizes shorter focusing times, outstanding tracking performance and smoother autofocusing during video shooting. And, because Live View shooting can be used in a manner similar to using the camera's viewfinder, the fast and smooth AF performance allows users to concentrate more attention on the subject and composing the photo when shooting.
Compared with the EOS Rebel SL1, which employs Hybrid CMOS AF II, the EOS 70D Digital SLR camera (scheduled to go on sale in September 2013), which is the first camera to feature Dual Pixel CMOS AF, not only achieves AF speed that is approximately 30 percent faster4 but also delivers improved Movie Servo AF tracking for continuously smooth focusing during video shooting, even when filming quickly moving subjects.
Because the outstanding AF performance of Dual Pixel CMOS AF is made possible with 1035 models of EF lenses (including many earlier models and models available outside of Japan), users can enjoy a wide range of photographic endeavors achievable with various lenses.
Recognizing great potential for the application of this technology across diverse product categories, Canon aims to promote its development efforts in the field of AF technology, targeting further advances to contribute to expanding the world of photographic expression.
Reference information
Phase-detection AF
With conventional phase-detection AF, the light that enters through the photographic lens is divided into two images. The difference in the focus point position between the two images is measured on a dedicated AF sensor rather than the image sensor itself, enabling the camera to determine the direction and amount of lens adjustment required to obtain proper focus. Because phase-detection AF enables fast focusing performance compared with contrast-detection AF, the technology is widely employed in digital SLR cameras, mainly for viewfinder shooting.
Dual Pixel CMOS AF employs the same measurement principle as a dedicated AF sensor, except that it is carried out directly with the image sensor. Its large coverage area enables smooth and reliable image-plane phase-detection AF for both still images and video with no reliance on dedicated AF sensors or contrast-detection AF.
Contrast AF
Contrast AF is an autofocus method employed in compact digital cameras and video camcorders, as well as conventional digital SLR cameras for Live View shooting. Because contrast is highest when an image is in proper focus, the camera analyzes the contrast information from the image on the image sensor, adjusting the lens until the maximum contrast value is reached. While contrast AF offers high focusing accuracy, it tends to require more time compared with phase-detection AF because the focusing components of the lens must be driven during AF measurement to find the point of peak contrast.
Hybrid CMOS AF and Hybrid CMOS AF II
Hybrid CMOS AF is an AF method employed in the EOS Rebel T5i Digital SLR camera and the EOS M digital camera that delivers enhanced focusing speed during Live View shooting and when shooting video. Combining fast phase-detection AF and high-accuracy contrast AF, Hybrid CMOS AF makes possible faster focusing performance than contrast AF alone, quickly measuring the subject distance using a dedicated phase-detection AF image element embedded in the CMOS image sensor and completing the process with extreme accuracy using contrast AF. The EOS Rebel SL1 camera features Hybrid CMOS AF II, which makes use of an imaging sensor that supports AF across a wide area spanning approximately 80 percent of the shooting area measured vertically and horizontally.
For more information about the development of Dual Pixel CMOS AF visit: http://usa.canon.com/gamechangeraf
About Canon U.S.A., Inc.
Canon U.S.A., Inc., is a leading provider of consumer, business-to-business, and industrial digital imaging solutions. With approximately $40 billion in global revenue, its parent company, Canon Inc. (NYSE:CAJ), ranks third overall in U.S. patents registered in 2012† and is one of Fortune Magazine's World’s Most Admired Companies in 2013. In 2012, Canon U.S.A. has received the PCMag.com Readers' Choice Award for Service and Reliability in the digital camera and printer categories for the ninth consecutive year, and for camcorders for the past two years. Canon U.S.A. is committed to the highest level of customer satisfaction and loyalty, providing 100 percent U.S.-based consumer service and support for all of the products it distributes. Canon U.S.A. is dedicated to its Kyosei philosophy of social and environmental responsibility. To keep apprised of the latest news from Canon U.S.A., sign up for the Company's RSS news feed by visiting www.usa.canon.com/rss and follow us on Twitter @CanonUSA.
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† Based on weekly patent counts issued by United States Patent and Trademark Office.
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1 Shooting area coverage of approximately 80 percent vertically and horizontally for the EOS 70D Digital SLR camera.
2 Contrast AF employed when using some older EF lens models, during magnified view, during Movie digital zoom and when using an extender.
3 Hybrid CMOS AF and Hybrid CMOS AF II, which combine phase-detection AF and contrast AF.
4 When used with an EF-S 18-55mm f/3.5-5.6 IS STM lens, based on a comparison by Canon. Disparity between AF methods may be small depending on lens used and shooting conditions. Disparity becomes greater when using older types of lenses.
5 As of July 2, 2013.
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