Not too long ago, using autofocus in video was slow, unreliable, and generally unacceptable. Companies have been working to make it viable for filmmakers, with Canon's solution being Dual Pixel CMOS Autofocus, which has generally been lauded for its performance. Here's a neat, short video on how it works.
For small crews or one-person productions, the ability to reliably autofocus when shooting can be a real lifesaver. Canon's Dual Pixel CMOS Autofocus was first introduced in 2013 with the 70D and now permeates most their line, from the mirrorless M6, through the t7i, 5D Mark IV, and C300 Mark II. What makes the system so fast and reliable is that it is essentially on-sensor phase detection autofocus, which gives the speed of a phase detection system (as opposed to contrast detection) and eliminates the accuracy errors introduced by having a separate autofocus sensor. Canon achieves this by splitting image pixels on the sensor in half and comparing the data between the two, which tells the camera both how far the lens is out of focus and the direction to focus in, which reduces hunting. The data from these half-pixels is then combined to create a full pixel for the purposes of image readout. Check out the video above for a visual guide to the system.
[via Canon Rumors]