Key takeaways:

• Rolling shutter is impressive (sub 10ms) in all video modes
• Readout rate is a fraction too slow to allow 120, hence the need for a slight crop
• Sensor has a dual conversion gain design, switching over 4.66 EV above base ISO

Rolling shutter

The a7S III takes a different approach to the one used in more stills-focused generalist cameras: rather than taking advantage of a high pixel count sensor to give more detailed footage, the a7S III uses just a handful more than the bare minimum required to shoot 4K at all. The great advantage of this approach is that it has less data to read out. Which in turn means less risk of rolling shutter.
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https://www.dpreview.com/reviews/sony-a7s-iii-initial-review/5