Dual Pixel Diode AF (autofocus) is a technique that was pioneered by Canon when it introduced the Canon EOS 70D back in 2013. The principle is rather simple, although extremely difficult to apply due to the precision required to manufacture the system.
The principle is this: each camera sensor pixel is in fact built with two independent photodiodes (light sensors) under a tiny microlens. For light sensing purposes, their output is combined, which means that they act is if it was a regular single-diode light sensing pixel.
For auto-focus, they act as independent light sensors which receive light from the same location in the scene, but with slightly different angles, because of the micro lens shape. Because of this, each dual pixel diode can be used as a phase-detection AF system.
Typically, image sensors with phase-detection AF have a number of pixels dedicated to AF only (note that most smartphones use lower-quality contrast-based AF, great Phase vs. Contrast AF comparison thread). They bring good AF performance, but their presence has a number of downsides, one of which is the fact that they replace light-sensing pixels dedicated to image sensing (vs. AF sensing). This makes it difficult to add more AF sensing pixels because it would be detrimental to color-sensing.
Dual Pixel Diode solves this problem by using color-sensing pixels to effectively do AF-sensing work. The whole image sensor can be built with dual pixel diodes so in theory, each and every pixel can now become an AF sensor as well, thus greatly adding to the number of AF points, and speeding up the focus time tremendously.
According to Canon, things such as continuous AF during movie recording (Servo AF tracking) is greatly improved by Dual Pixel Diode AF. [Canon brochure on Dual Pixel CMOS AF (PDF)]
In conclusion, Dual Pixel AF has been proven to:
- Accelerate AF operations in general, by up to 80% compared to prior phase-detection systems
- provide smoother video operation AF / Servo AF
- improve subject tracking performance