Canon EOS 70D review – Dual Pixel CMOS AF
The standout feature of the Canon EOS 70D is the unique Dual Pixel CMOS AF system built into its 20.2-million-pixel sensor. This technology is Canon’s method for performing on-sensor phase-detection focusing. Each of the 20.2 million pixels that make up the images are produced from two photodiodes on the sensor.
The reason for having pairs of photodiodes is that any difference in the electrical readout between the pair will indicate that the image is not in focus. Depending on the exact difference between the readout of the photodiodes, the camera’s processing system can calculate how much the focus of the lens needs to shift to make each readout match. When this is achieved, the image is in focus.
As the signal from these photodiodes is used to create a single pixel in the image, Canon insists that the system does not degrade image quality in any way.
Although the entire surface of the sensor uses the same dual photodiode arrangement, the Dual Pixel CMOS AF system only covers 80% of the image frame. This is because lens distortions could affect the accuracy of focusing at the very edges of the frame, so coverage has been restricted to prevent it.
Of course, on-sensor phase-detection systems are primarily to allow the use of autofocus when shooting using live view or video mode, and in theory they should provide a faster method of focusing than contrast-detection AF systems. However, contrast-detection AF has come on leaps and bounds in the past few years as camera processing power and actuator motors in lenses have become more powerful. One area where contrast detection is still some way behind phase detection, however, is when shooting a moving subject in continuous AF mode.
Images: By analysing the difference between the output of each photodiode, the lens can be refocused to the correct position
To find out just how good the Dual Pixel CMOS AF system really is, see the Autofocus section.