The R5 Mark II’s electronic shutter reaches 1/64000s. In practice, 1/8000s is the useful ceiling for most fast-motion subjects — athletes, birds in flight, children running — because beyond that shutter speed, the light gathering falls off steeply and ISO requirements climb into ranges where noise management becomes the dominant concern. At 1/8000s with the RF 85mm f/1.2L and ISO 1600 in full sun, you get a completely motion-frozen frame. A sprinter at 10 m/s travels 1.

Panning is the technique of tracking a moving subject with a slow shutter speed so that the subject is relatively sharp while the background blurs into horizontal streaks. At 1/60s, a cyclist or vehicle moving across the frame renders with context — the motion of the world around the subject made visible — rather than as a freeze-frame extracted from its environment. The technique is old. What is new is doing it at 400mm.