How can on-sensor PDAF work when left- and right-masked pixels aren’t adjacent?
Asked 4/1/2021
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I understand the basic idea of phase-detection autofocus: light from opposite sides of the lens is compared, and when the two views line up, the subject is in focus. But with on-sensor PDAF, the masked or split detection pixels are often spread out across the sensor rather than sitting side by side. If the subject is in focus, shouldn’t light from a scene point fall on one exact pixel location? How can separated left- and right-looking PDAF pixels still produce matching information for focus detection?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
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How on-sensor phase detection is accomplished varies somewhat. It can be pixels where the micro lenses are oriented in opposite directions. It can be multiple *pixels under a single micro lens with a baffle between them. And it can be partially masked pixels (but that example image isn't very good IMO).
And there's probably some arrangement yet to be designed or I don't know about; but they all work based on the converging virtual images as shown in your first two diagrams.
And you are right; however it is accomplished, when the left/right (or upper/lower) virtual images merge as a single focused image there is no longer a phase difference. At that point a mirrorless camera must switch to contrast detection for any changes/refinements. Of course, what is considered "focused" in this sense depends on the resolution of the sensor/size of the PDAF points. E.g. if the two virtual images are falling on two separate photodiodes, which are binned as a single pixel in the output, then they are maximally "in focus" (combined as a single image) even though there is still a separation.
And even if both virtual images are perfectly aligned that does not mean that the camera cannot monitor the PDAF focus points in order to detect when a phase difference reoccurs (and then correct for that).
On-sensor PDAF is very much the same as using a split prism viewfinder; when using a split prism viewfinder you can see when the left/right virtual images are not combined as a single image (in focus), you can see when they are, and you can see when they separate again. And likewise, the on-sensor PDAF is very dependent on the light/image falling on it just as the split prism is... this is all quite different from the DSLR's dedicated PDAF system (which uses multiple real images).
*they are now starting to distinguish the difference between photodiode/detector and pixel/picture element when multiple photosites are binned in the sensor output.
EDIT TO ADD I found this good reference on the history of focusing in cameras. It covers the physics of using ground glass (diffused focus screen), rangefinders, split prisms (phase); and discusses their implementations into autofocus. https://www.pointsinfocus.com/learning/cameras-lenses/brief-history-focusing/
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
5y ago
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On-sensor PDAF does not usually compare a single scene point on two neighboring pixels. It compares two partial views of the image formed by opposite sides of the lens and looks for phase differences in lines/edges of contrast.
The PDAF elements can be made in different ways: partially masked pixels, split pixels under one microlens, or microlenses biased in opposite directions. The simplified diagrams online are often not literal sensor layouts.
What matters is that one set of photosites samples light from one side of the lens and another set samples light from the opposite side. The camera compares their signals in the same row/column region to see whether contrast features are shifted relative to each other. That shift tells both whether the image is out of focus and which direction to move focus.
When the image is in focus, the two partial views align and the phase difference goes to zero. At that point PDAF gives little or no directional error signal, so fine final confirmation/refinement may rely on contrast detection or other processing.
So adjacency is not the key requirement; corresponding sampling of opposite lens halves over the same image area is.
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