Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 391

[ 3] Movie Servo AF speed You can set the Movie Servo AF's AF speed and its operation conditions. The function is enabled when using a lens supporting slow focus transition during movie recording.* When active You can set [Always on] to have the AF speed take effect at all times for movie recording (before and during movie recording) or set [During shooting] to have the AF speed take effect only during movie recording. AF speed You can adjust the AF speed (focus transition speed) from the standard speed (0) to slow (one of seven levels) or fast (one of two levels) to obtain the desired effect for the movie creation. * Lenses supporting slow focus transition during movie recording USM and STM lenses released in and after 2009 are compatible. For details, refer to the Canon website. Caution With certain lenses, even if you adjust the AF speed, the speed may not change. 391