Autofocus Systems Part IV

  • Dual Pixel CMOS AF

Although only being another step in the development of on-sensor phase detection, Dual Pixel CMOS AF is considered revolutionary. Dual Pixel CMOS AF was first introduced in 2013 with the release of the Canon EOS 70D. Again, this sophisticated technology is based on phase detection performed on the image sensor directly. However, the significant difference between Dual Pixel CMOS AF and the previous developments is that each new pixel can record both halves of the incoming light rays simultaneously and evaluate both signals individually.Therefore, each pixel consists of two sub-photodiodes half the size of a regular photodiode. During autofocus performance, each sub-photodiode can be read out individually by the autofocus circuitry. On the other hand, during image-capture, both sub-photodiodes are combined to collect as much light as possible. The figure below shows the new structure of pixels used for Dual Pixel CMOS AF.

Dual Pixel Photodiodes

There are numerous advantages of this new pixel structure. Firstly, no cover plates are required to shield one half of a photodiode and therefore the intensity of light falling on a photosite is not reduced by absorption. Secondly, as the pixel sensitivity remains unchanged by the new design, the density of specialized pixels can be increased so the image sensor is filled completely with these phase detection sensors. The figure below shows the arrangement of pixels on the image sensor.

Dual Pixel CMOS AF Sensor Layout

The increased resolution of specialized phase detection cells is what allows Dual Pixel CMOS AF to perform phase detection with extreme accuracy as the phase difference can be brought to coincidence by fine increments. For that reason, phase detection can be used as the only autofocus system and contrast detection is not required. Looking at the diagram below, the increased resolution of specialized cells is clearly visible and explains why the phase-matches can be determined more precisely.

Dual Pixel CMOS AF Phase Detection

With contrast detection made obsolete, the speed is drastically increased. On-sensor phase detection is still not as fast as conventional phase detecion systems because it takes longer to evaluate the signal of all Dual Pixels while for conventional PD systems the sensor is reduced to a minimum. Still, Dual Pixel CMOS AF is a great invention and takes focusing during live-view or video shooting to the next level.

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