I can't find any information about whether the sensor read rate increases in crop mode and hence rolling shutter decreasing. This would show that only the crop lines on the sensor are read vs the full sensor. If the sensor read rate doesn't change from full to crop then the AF system can't be faster - or have I misunderstood?
It's not so much about the readout time as it is about the processing time. If the camera only need read 63% of the sensor's lines for AF, then it can do so at a higher rate per second for each line used than if it is scanning 100% of the sensor's lines. It can cycle through only the middle 63% 160 times in the same amount of time it would take to cycle through the full sensor for 100 times.
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