Canon EOS-70D: the real advantage

Unless you’ve spent the last few weeks on an uninhabited island, you probably know that Canon has announced the EOS-70D. Hundreds of websites and blogs have published the press release or wrote their own comments, but many of them seem to be missing the point. The EOS-70D has this new ‘Dual Pixel AF system’ that enables phase detection autofocus over almost the entire sensor during video filming. While this system is fast and accurate, that still isn’t the most important point of having phase detection autofocus. Most other cameras use contrast detection and that can also be fast and accurate nowadays. The major advantage of using phase detection however is when you focus during video taking, for example to switch focus from one speaker to the other. Contrast detection works by measuring the contrast of the subject while the lens is focussing. When the highest contrast is achieved, the subject is in focus. Unfortunately however, the camera cannot know in advance where that point is. It has to let the lens travel beyond the point of focus, so it can see that the contrast starts to diminish again. Only then it knows where the focus was and can adjust the lens. That means that in the video you will see the camera go beyond the correct focus and then go back. With phase detection you do not have this problem and that is the most significant advantage of phase detection AF during video.