The Dual Pixel CMOS AF Autofocus system of EOS 70D

On board of the new Canon EOS70D – announced today – has made the debut of a new autofocus technology, developed by Canon, called ´Dual Pixel CMOS AF´. It promises a significant improvement of the autofocus performance. The sensor of Dual Pixel CMOS AF is based on phased detection, and is designed to supply a smooth focus and high performance in the in the video shooting and a fast autofocus when photos are taken in Live View mode. It’s ideal for the creation of professional looking ‘pull-focus’ effects, and to keep in focus a subject moving towards the camera, while maintaining the background blurred.

EOS 70D

EOS Movie allows capturing movies with a reduced depth of field, but certain level of skill is necessary to obtain an accurate focus. Dual Pixel CMOS AF has been developed to make it easier to record video and control the focus when the subject moves across the frame. With Dual Pixel CMOS AF, management and control of the focus in video mode is faster and easier than ever, since it ensures that there are no errors when focusing while the action takes place, and the objects are moving inside the frame. Dual Pixel CMOS AF also comes in handy when taking photographs in Live View mode(framed with the LCD display): fast autofocus, with similar speed to the one available when using the viewfinder.

How does the Dual Pixel CMOS AF work?

In the new CMOS sensor of 20.2 million pixels, all the pixels in the sensor’s surface are formed by two single photodiodes, for a total of 40.3 million photodiodes. These photodiodes can be read either separately in order to obtain phase-detection autofocus, or together as pixels of imaging. Unlike other autofocusing methods based on sensors that assign both the autofocus and imaging functions to the same pixel sensor, or which are exclusively based on contrast detection.
Since no supplementary processing is required for the pixels of imaging dedicated to AF, one can count both on a fast focusing and on maximum image quality when shooting photographs and video. This new technology enables EOS70D to take advantage of Dual Pixel CMOS AF on 80% of the frame.

Dual Pixel CMOS AF Autofocus system

To perform the phase detection on the image plane, EOS 70D reads the signals from the photodiodes left and right of an individual pixel independently, then it calculates the phase difference of the two images in parallax. This technique allows EOS 70D to predict the necessary position of the lens to obtain a precise focus, and subsequently moves the lens to this position. In comparison, the more traditional contrast AF methods search for the correct position by moving the lens back and forth until it achieves the precise AF, process which slows down the overall autofocusing speed and hinders the smoothness of the maneuver.

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