"I don't think that the majority of customers knew that SLRs had both [focus systems]," explains Nikon Senior Technical Manager, Steve Heiner. "We hear 'Why don't I have the same focus in Live View?' and it's because contrast detect can be done with the sensor itself." Customers shelling out a lot of money for a high-end SLR end up getting a powerful phase-detect system and fast focus with pro-level lenses, but then they see that performance slow down when shooting video or in the mirror-up Live View mode.
Once the camera's mirror is locked up, as is necessary for video and live view, the phase-detect sensor can't get the necessary image to evaluate; so not only does the camera focus more slowly, but it loses the predictive focus capabilities of the phase-detect system.
While contrast detection can accurately lock onto a stationary subject, it's not able to predict where the subject will be next, and that's a problem for continual autofocus on a moving subject.
Remember that phase-detect autofocus can tell if the subject is out of focus because the lens is front- or back-focused, so with a little bit of math, the camera can guess where the subject will be as it continues to move.
Here's an example. Let's say a subject is moving across the field of view and phase detect locks. A second later, it reevaluates and sees that the subject is out of focus by 10 feet, so it corrects and refocuses. Another second, and the car is 10 feet farther out of focus. The car is moving 10 feet per second, so now it can predict what the focus should be in four seconds (40 feet).
Contrast detection can't do this predictive focus. It can perform superfast continual focus, but it can't track an object moving across the scene the way phase detect can.
A New BreedSo, up until a few years ago, the state of autofocus was this: Predictive autofocus is the fastest system available, and it's able to track objects with predictive focusing.
But a funny thing happened on the way to a mirrorless world—technologies changed and improved. By designing a digital-specific system, the Four Thirds partners were able to make smaller, lighter cameras and lenses that threw away some of the SLR rulebook.