The latest patent filed by Nikon in Japan concerning AF is an interesting one. Basically it describes an image sensor that performs with a global shutter during AF (i.e. composing) but switches to a rolling shutter for the actual image taking.
To understand why that would be desirable you have to look at what’s happening currently with many phase detect systems in mirrorless cameras. The old Aptina/Nikon method developed in the Nikon 1 cameras and still used by Nikon, Sony, and others is to use rows of sensor data that can be pulled off the image sensor simultaneously. In the current cameras, that’s every 12th row. Effectively, there’s a global shutter for every 12th row but a rolling shutter for all rows during image taking.
The reason you need a global shutter for focus information is that with moving subjects, the subject may have moved slightly by the time you read the next rolling shutter group of photosites. Focus accuracy can be compromised when relying upon rolling shutter information. Meanwhile, focus accuracy is compromised by the current system because it really only sees detail on the long axis (the rows of focus-enabled photosites, but with big gaps between rows).
The reason you don’t want a global shutter for the image data is that electronic noise of various sorts will compromise the dynamic range and photosite data integrity.
I keep seeing clues that some future Nikon camera—likely the Z9—will use all photosites for focus and introduce a focus system with better vertical data discrimination.
