Released as the successor model of the EOS 60D, the EOS 70D is equipped with a new, revolutionary technology, "Dual Pixel CMOS AF." The following is an interview with the development team on the user benefits of the EOS 70D. Here in [Part 2], the Dual Pixel CMOS AF feature is introduced in details. This new function may change the future trend of Canon's DSLR. Also, find out more about EOS 70D's body structural design. (Based on the interview held in August 2013) (Interviewer: Ryosuke Takahashi / Photos by: Takehiro Kato)
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(Back row, from left)
Shinichiro Yano, Camera Development Center/ Yohei Yamanaka, Camera Development Center/ Hideaki Ohshima, Camera Development Center/ Atsushi Watanabe, Photo Products Group
(Front row, from left)
Kazuhiko Miyahara, Design Center/ Futoshi Hirai, Camera Development Center/ Hiroshi Miyanari, Camera Development Center/ Saori Yasukawa, Photo Products Group
Giving a "condensed" and "robust" feel to the compact body design
― Next, could you tell me more about the features of the new CMOS sensor?
Miyanari We have increased the number of effective pixels from about 18 megapixels in the past to about 20.2 megapixels. In conjunction with this change, we have altered the pixel size to 4.1μm. Although reducing the pixel size is a disadvantage, as it reduces the amount of light each pixel can gather, the improvements we have made on the EOS 70D enabled us to expand the upper limit of the standard ISO speed by one stop compared to the EOS 60D. A sufficiently wide dynamic range has also been secured.
EOS 70D is equipped with approx. 20.2 megapixels CMOS sensor. Despite the comparatively small 4.1μm pixel size, EOS 70D realizes outstanding high ISO performance with ISO 12800 as its maximum standard range.
― Can you give a more detailed description of the design and material of the camera body?
Hirai The external casing of the EOS 70D makes use of the same resin material as that of the EOS 60D. An electromagnetic-shielding resin material is used for the front and rear shells. As the EOS 70D is built in with a wireless LAN unit, a resin material with no electromagnetic shielding properties is employed only for the top shell.
― Where is the wireless LAN unit built into?
Hirai It is located under the Mode Dial.
The Wi-Fi unit of EOS 70D is built in under the Mode Dial. Resin material with no electromagnetic shielding is used for the top cover to prevent any influence.
― Which points did you focus on when designing the camera body?
Miyahara We came up with design ideas based on the theme to further enhance the "condensed" and "robust" feel of the body. The EOS 70D is more compact in size than the EOS 60D, but simply downsizing the body would make the camera look less sturdy. So, we tried to bring out the condensed feel of the entire body using a sharper curved surface, while expressing a sense of robustness that is suitable for a middle-class DSLR camera. Reducing the size of the body usually leads to deterioration in the hold of the grip. To prevent this, we retained the shape of the EOS 60D at the area where the middle, ring, and little fingers rest, while adopting a gradual curve for the entire grip surface so that the index finger would fit to it naturally. Also, we employed a sharper design for the contour on the two sides of the built-in flash housing. To put it more specifically, we added a slope to the contour to bring out the edge, thus stressing the shape that runs from the front to the back.
The body design of EOS 70D aims a "condensed" and "robust" feel. The grip design is also considered to maintain good hold while realizing a small body design.
― What were the efforts made in designing parts such as the button layout and the Mode Dial?
Miyahara Besides introducing a Mode Dial that can be rotated 360°, we adopted the same pyramid pattern as that of the EOS 6D on the side of the Mode Dial. The rubber of the dial is molded in two colors to create an elegant feel, and to enhance its operability at the same time. We have also relocated all the icons for the scene modes in the Mode Dial under SCN. Meanwhile, the button layout is now closer to the design of the EOS 6D. The frequently-used Erase button is now shifted to the right side of the body, allowing smooth operation using the right hand alone. Although the Quick Control Dial and Multi-controller now come in a smaller diameter compared to the EOS 60D, we have also revised the shape of the dial and the number of pitches to enhance their appearance and operability. As for the Multi-controller, many efforts were made to adjust its height as well as the feel when it is pressed.
The button layout of EOS 70D is close to that of the higher-end model EOS 6D. The erase button is placed on the right side to ease operation with one hand.
― Are there any advancements made in the Vari-angle LCD monitor, such as its performance or clarity?
Hirai The operating angle of the Vari-angle LCD monitor is the same as that of the EOS 60D. Although the size of the LCD panel and the number of dots remain unchanged, a Clear View II LCD Monitor has been employed on the EOS 70D. Also, we adopted a borderless design that integrates the outer frame with the LCD panel. Furthermore, we have added a touch-screen function as well as an anti-stain coating on top of the anti-reflective coating. Doing so prevents the surface from being stained by sebum, and makes it easier to wipe off any dirt.
Clear View LCD
- Protective cover
- Anti-reflection Coat
- LCD panel
Clear View LCD II
- Smudge-resistant coat
- Anti-reflection coat
- Protective cover
- Photoelastic material
- Touch screen electrode
- Photoelastic material
- LCD panel
The Vari-angle Clear View II LCD Monitor employs anti-stain coating to support the touch-screen function.
"Hunting-free AF" Dual Pixel CMOS AF
― Next, I would like to know about the mechanism of the Dual Pixel CMOS AF, the most distinctive feature of the EOS 70D.
The Dual Pixel CMOS AF logo. This new technology is said to change the trend of future Canon DSLRs.
Watanabe Regardless of the type of product, it is not unusual that the entire market may be revolutionized by the advent of a single technology or product. If we look at the computer or mobile phone industry, it would be easy to find such instances. In the context of Canon's technologies and cameras, some examples would be the AE-1 that was released in 1976, and the EOS 650, the first model of the EOS system launched in 1987. In the digital camera era, Canon has also been expanding its lineup of entry-level EOS models, which contributed to the widespread use of digital SLR cameras. Seen in this light, the Dual Pixel CMOS AF can be regarded as the technology that changes the future trends of Canon's DSLRs, and the first model to be equipped with this technology is the EOS 70D. You can say that the Dual Pixel CMOS AF has ushered in a new age for Live View shooting.
― What is so amazing about the Dual Pixel CMOS AF compared to the conventional AF function?
Watanabe First of all, the Dual Pixel CMOS AF does not hunt for focus. With it, AF during Live View shooting can be performed at the same speed as during viewfinder shooting, allowing you to create smooth movies. The second feature is Canon's unique technology that makes possible a total of about 40.3 million photodiodes on the image sensor. I will talk about the details later, but basically this technology allows all the pixels on the image plane to perform both the functions of phase-difference detection as well as imaging. By utilizing this technology, we believe users will be able to enjoy "Speed, Comfort, and Quality" during both viewfinder and Live View shooting, which is what the EOS series seeks to achieve.
Dual Pixel CMOS AF & Hybrid CMOS AF
Hybrid CMOS AF
Blue & Red: Pixels capable of phase-difference AF
Pixels that are capable of phase-difference AF are spread out (Each pixel of Hybrid CMOS AF does not function as two independent photodiodes)
Dual Pixel CMOS AF
Blue: Photodiode A
Red: Photodiode B
Phase-difference AF can be performed by all pixels
Dual Pixel CMOS AF performs focusing based on the difference in the signals from the two photodiodes. Meanwhile, Hybrid CMOS AF performs focusing using only one photodiode, but final focusing cannot be done solely with phase-difference AF.
― What kind of technology is the numerical value of about 40.3 million photodiodes based on?
Miyanari Considering the balance between the imaging performance and the phase-difference AF function, the nominal value for the number of effective pixels is generally taken to be about 20.2 million pixels, after rounding off the actual count of 20.16 million pixels. Since there are now two photodiodes for each pixel, 40.3 million is simply the value after doubling 20.16 million and rounding off the figure. In order to build two photodiodes into a single pixel, we have optimized the miniaturization technology, pixel structure, readout speed from the image sensor, and the phase-difference signal processing system.
- Photodiode A
- Photodiode B
Each photodiode is capable of capturing light independently
Structure of CMOS sensor
Although there is no line that separates the left and right photodiodes, they function separately to achieve focus. During shooting, image signals are stored in the same way as conventional image sensors. In other words, each pixel plays two roles at the same time.
― How is it different from the existing systems?
Miyanari All the pixels that are built into the image sensor of the EOS 70D are equipped with both the functions of phase-difference AF and imaging. However, the applicable area of the Dual Pixel CMOS AF is within about 80% of the image sensor area. In the diagram, red and blue colors are used to differentiate the two photodiodes inside a pixel, but they are not colored in real life. As with the AF sensor used for viewfinder shooting, the two "colored" photodiodes read out the amount of difference in the AF signals, which is equivalent to the amount of defocus in the subject. Based on this difference, the amount of lens drive is calculated for focusing to be performed. Also, the function of the conventional imaging pixel is now fulfilled by combining the signals from the two photodiodes.
Procedure of focusing during AF
1. Signal is detected by each photodiode.
2. The difference between the signals (amount of blur) within the AF point is calculated to drive the lens by the required amount.
Blue line & Red line: Photodiode signal
3. Driving the lens.
4. Unlike conventional contrast AF, this enables obtaining sharp focus right away.
Two separate photodiodes built in a single pixel reads out the amount of difference in the AF signals. Then calculates the amount to drive the focus lens to establish focus. The mechanism is the same with a conventional AF sensor used for viewfinder shooting and so shares the characteristic of "hunting-free" AF.
― Are the pixels physically divided into two photodiodes?
Miyanari No. There is no line that physically divides the red and blue photodiodes. Inserting a slit creates a dead zone, and causes the pixel to become defective. This is where the technological breakthrough lies. By making possible what wasn't before, we were able to develop an all-new imaging pixel.
― How is it different from the phase-difference AF during viewfinder shooting?
Miyanari The most distinctive difference is the absence of an optical system used exclusively for AF. Unlike the phase-difference AF sensor for viewfinder shooting, Dual Pixel CMOS AF does not disperse light using an AF optical system to obtain the information necessary for phase-difference AF. Instead, AF signals are extracted directly from the two photodiodes of the CMOS sensor to detect the amount of defocus in the subject.
― A base line exists in the AF sensor for phase-difference AF during viewfinder shooting. Does Dual Pixel CMOS AF make use of the same concept?
Miyanari For phase-difference AF during viewfinder shooting, a few types of base lines with different lengths are built into the AF sensor for focusing to be performed. The type of base line to use is determined by the maximum aperture value of the lens being attached to the camera. In the case of Dual Pixel CMOS AF, the length of the base line is not fixed in advance; it changes with the aperture of the lens. In other words, the pixels form a link to function as a line sensor, so multiple pixels are used for focusing.
― So, can I say that the concept of vertical-line and horizontal-line detection in viewfinder shooting is also not applicable to Dual Pixel CMOS AF?
Miyanari Yes, it is applicable. You can think of the phase-difference AF of the image sensor that enables Dual Pixel CMOS AF as vertical-line detection AF with horizontally-arrayed line sensors. I can't provide you with the details, but they are capable of detecting horizontal lines that are at a slight angle.
- Image sensor size
- Dual Pixel CMOS AF area
Diagram of the Dual Pixel CMOS AF sensor layout. It can be said that the Dual Pixel CMOS AF is a vertical-line detection AF with horizontally-arrayed line sensors. It can also detect lines at slight angle from horizontal state.
― How well does it handle shooting at dimly-lit locations, and how does it fare compared to the performance of phase-difference AF during viewfinder shooting?
Miyanari The crucial point here is the frame rate of the video in the Live View display. In other words, it is not possible to expose the pixels on the image sensor to light for a duration longer than the predetermined frame rate. In this sense, phase-difference AF during viewfinder shooting, which is not affected by the frame rate, has a greater advantage in principle. However, while it is difficult to distinguish the subject through the viewfinder at a pitch dark location, the Live View function allows you to perform focusing while viewing the subject, so if we take the entire shooting process into consideration, we could also say that Dual Pixel CMOS AF has its advantages too when shooting at dark locations.
― Thank you for the clear explanation and for taking time off your busy schedule today.