In summer 2013, a new DSLR made its appearance, which gives a whole new definition to Live View AF. Below is an introduction of the new EOS 70D, the first model to be equipped with the groundbreaking AF technology. Here in [Part 1], let's look at the mechanism of the Dual Pixel CMOS AF and its benefits. (Reported by: Ryosuke Takahashi)
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The surprising debut of the EOS 70D
EOS 70D, a revamped model, has made its appearance some three years after the release of the EOS 60D, which has gained much popularity among a wide range of users from beginners to professionals. Represented by the EOS 70D, the "EOS two-digit models" belong to a distinguished lineage that has shaped the history of Canon's digital SLR cameras. Ever since the debut of the EOS D30 in 2000, this class of cameras has ushered in a host of breakthroughs. With the enhancements made to the performance of the "EOS three-digit models" in recent years, however, they have caught up with the two-digit class in popularity. Against this backdrop, the EOS 70D has been launched with the aim to restore the name of the EOS two-digit cameras, which are characterized by their mobility and professional quality.
In addition to the enhanced shooting performance by incorporating the superior features of the higher-end models, the EOS 70D also comes with an advanced AF function. The advent of the "Dual Pixel CMOS AF" is likely to revolutionize the common notion of AF shooting using the Live View function. Similar to the impact when Canon first introduced the CMOS sensor on a DSLR camera, the technology of the Dual Pixel CMOS AF will appeal not only to photo enthusiasts, but also camera fans. The revamp of the Live View AF function has contributed to a tremendous improvement in the ease of operating the shooting functions on the EOS 70D.
|Effect pixel count||Approx. 20.2 megapixels||Approx. 18.0 megapixels|
|Image processor||DIGIC 5+||DIGIC 4|
|Viewfinder coverage||Approx. 98%||Approx. 96%|
|Viewfinder magnification||Approx. 0.95x||Approx. 0.95x|
|No. of viewfinder AF points||19 (all cross-type)||9 (all cross-type)|
|AF area selection mode||Single-point AF (Manual selection), Zone AF (Manual zone selection), 19-point automatic selection AF||Single-point AF (Manual selection), 9-point automatic selection AF|
|Live View AF||Dual Pixel CMOS AF system, Contrast-detection AF system (Face+Tracking, FlexiZone-Multi, FlexiZone-Single), Phase-difference detection with the dedicated AF sensor (Quick mode), Manual focusing||Live mode, Face detection Live mode, Quick mode, Manual focusing|
|ISO speed||ISO 100 to 12800 (expandable to ISO 25600)||ISO 100 to 6400 (expandable to ISO 12800)|
|Continuous shooting speed||Up to approx. 7 shots per sec.||Up to approx. 5.3 shots per sec.|
|Size (W x H x D)||Approx. 139 × 104.3 × 78.5mm||Approx. 144.5 x 105.8 x 78.6mm|
|Weight (CIPA Guidelines)||Approx. 755g||Approx. 755g|
|Click here for detailed specification||Click here for detailed specification|
Live View shooting comes to a new era with the Dual Pixel CMOS AF
Similar to viewfinder shooting, the Dual Pixel CMOS AF enables focusing to be performed with only phase-difference AF during Live View shooting. The key distinction compared to contrast AF, an AF method employed on many cameras for Live View shooting, is that the Dual Pixel CMOS AF is able to grasp the position of the subject accurately at the time focusing starts.
Mechanism of Dual Pixel CMOS AF During AF
1. 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
Information on the subject is captured by each of the two photodiodes located inside every pixel of the image sensor. Based on the difference in the signals obtained from the two photodiodes, the distance to the subject is calculated, and the direction to drive the lens is determined. In other words, the goal is clearly defined at the time focusing starts.
2. Driving the lens.
Based on the detected data, the focusing lenses are driven to perform focusing. Just like contrast AF, the focusing lenses are moved to the point of focus without hunting once focusing starts, allowing focus to be established quickly.
3. Unlike conventional contrast AF, this enables obtaining sharp focus right away.
While the Hybrid CMOS AF switches automatically to contrast AF near the focal plane, focusing is complete with only phase-difference AF in the case of the Dual Pixel CMOS AF. This allows Live View AF shooting to be performed in the same way as viewfinder shooting.
As contrast AF sets the focus by searching for the contrast peak on the imaging surface, AF speed tends to slow down considerably at the final stage of focusing. Not only so, there are also scenes where focus cannot be achieved. Phase-difference AF, on the other hand, determines where to stop the focusing lens group at the time focusing starts, and thus AF speed does not drop near the focal plane.
Comparison of AF Speed (Conceptual Diagram)
Phase-difference AF during Viewfinder (Red)
Dual Pixel CMOS AF (Blue)
Focusing is possible with only imaging-surface phase-difference AF
Hybrid CMOS AF (Green)
Imaging-surface Phase-difference AF + Contrast AF
As the Dual Pixel CMOS AF does not switch to contrast AF near the focal plane, AF speed is twice as fast compared to that of the Hybrid CMOS AF (based on Canon's data). As can be seen in the graph above, the focusing lenses move to the target in almost a straight line.
Two photodiodes are built into each pixel of the Dual Pixel CMOS AF. In other words, phase-difference AF can be performed with every pixel within the AF focusing range acting as an "eye." At the same time, the two photodiodes also function as an image sensor. During imaging, the photodiodes work in unison to read out the image signals for the image data to be generated by the image processor. Noted for its excellent functionality, such as the AF speed, the key of the Dual Pixel CMOS AF lies in the structure of the image sensor, where the photodiodes are capable of performing both focusing and imaging. This makes it possible for all the pixels on the image sensor to be equipped with the AF properties.
Newly-developed CMOS Sensor
Imaging Area (Red)
Pixels equipped with both the functions of phase-difference AF and imaging are built in throughout the entire CMOS sensor.
Applicable Area of Dual Pixel CMOS AF (Blue)
Phase-difference AF is possible within about 80% (vertical) x 80% (horizontal) of the imaging area.
With a resolution of about 20.2 megapixels, each pixel on the newly-developed CMOS sensor is built in with two photodiodes. Phase-difference AF can be performed within about 80% (vertically and horizontally) of the imaging surface. Unlike the Hybrid CMOS AF, the photodiodes are not located away from each other, thus enabling more precise focusing.
As the Dual Pixel CMOS AF operates based on the same principles as the phase-difference AF system, it supports AF shooting by a total of about 103 lenses* in the EF series, both old and new. Additionally, it is compatible with all the actuators for driving the focusing lenses, including the responsive STM (stepping motor), DC motor, ring USM, and micro USM. In particular, for lenses that need to drive large and heavy lens elements for focusing, such as a large-diameter lens, contrast AF is not suitable, as it needs to move the lens elements forward and backward at the final stage of focusing. Since the entire EF lens series is designed for phase-difference AF, it only makes sense to drive the lens elements with the same method. In short, the Dual Pixel CMOS AF can be likened to a highway that facilitates the movement of the EF lenses during Live View shooting.
* As of July 2, 2013. The number includes all previous models as well as those available only in a limited area.
Born in Aichi in 1960, Takahashi started his freelance career in 1987 after working with an advertising photo studio and a publishing house. Photographing for major magazines, he has travelled to many parts of the world from his bases in Japan and China.