Canon has developed Dual Pixel CMOS AF, a new autofocus technology for the EOS 70D DSLR (see previous story).
A phase-detection AF technology conducted directly on the image sensor plane, it employs a CMOS sensor on which all of the effective pixels are able to perform both imaging and phase-detection AF simultaneously to achieve what the company terms “dramatically improved” AF performance over prior EOS cameras during Live View and video shooting.
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 a 70D’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 area through phase-detection AF until final focus is achieved.
Compared with earlier generations of Canon’s image-plane phase-detection AF, Dual Pixel CMOS AF realizes shorter focusing times, outstanding tracking performance and smoother autofocusing during video shooting, says Canon.
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, suggests the company.
Compared with the EOS Rebel SL1, which employs Hybrid CMOS AF II, the 70D (scheduled to go on sale in September), which is the first camera to feature Dual Pixel CMOS AF, not only achieves AF speed that is approximately 30 percent faster, says Canon, but also delivers improved Movie Servo AF tracking for continuously smooth focusing during video shooting, even when filming quickly moving subjects.
Apparently the new AF tech works with 103 EF lenses, including many earlier models and models available outside of Japan.
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 DSLRs, 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 is an autofocus method employed in compact digital cameras and video camcorders, as well as DSLRs 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 is an AF method employed in the Rebel T4i DSLR and the EOS M compact-system 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 Rebel SL1 features Hybrid CMOS AF II, which makes use of an imaging sensor that supports AF across a wide area spanning approximately 80% of the shooting area measured vertically and horizontally.