Oh sorry I misunderstood your point. That is so the AC can get an accurate measurement from the sensor/gate to the focus point, which they then mark on their follow focus. Often it’s a few marks because a shot will have different items of interest or the camera is moving. When you’re shooting on film this is particularly important, but a lot of people also use it for digital sensors so you’re not eyeballing it or depending on focus peaking (though plenty of quality focus pullers can absolutely eyeball it most of the time). Boils down to the shoot you’re on and the process they want, or the importance of the shot, such as something with huge explosions and a flipping car where are you want to reduce any possible margin for error.
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Oh sorry I misunderstood your point. That is so the AC can get an accurate measurement from the sensor/gate to the focus point, which they then mark on their follow focus. Often it’s a few marks because a shot will have different items of interest or the camera is moving. When you’re shooting on film this is particularly important, but a lot of people also use it for digital sensors so you’re not eyeballing it or depending on focus peaking (though plenty of quality focus pullers can absolutely eyeball it most of the time). Boils down to the shoot you’re on and the process they want, or the importance of the shot, such as something with huge explosions and a flipping car where are you want to reduce any possible margin for error.