For successful autofocus during video, a pro-grade video camera should be able to accomplish the following tasks:
The autofocus system should be capable of high, repeatable accuracy to achieve optimum sharpness over a range of conditions.
Adjustments must be smooth between focus points and free of hunting back and forth (oscillation). Even rapid changes can be detrimental to the viewing experience. This differs from AF in stills capture where smoothness is irrelevant.
Subject tracking is a highly attractive and sought-after feature (effectively automatic focus pulling – one of the most difficult and demanding aspects of professional filmmaking and video capture); one that contrast detection AF systems found in most mirrorless / DSLR cameras has been unable to achieve with any real success.
The autofocus system has to locate subject distance without losing, or ‘dropping’ focus. Contrast detection systems are prone to re-focusing leading to distracting ‘oscillation’.
On paper, the Canon EOS 70D sounds promising, especially when equipped with the firm’s new STM lenses. The Canon should also perform well in low light AF, where the new system works in light levels as low as EV0 (@ ISO 100) and at apertures down to f/11.
On the other hand, both the image sensor and the separate phase detection AF sensor module of the Sony SLT-A77 loses some of the light with the adoption of a semi-silvered mirror, resulting in slightly higher noise levels in images.
Be that as it may, the phase detection AF system is rated to -1EV (@ ISO 100), though the full-time video AF mode is restricted to a maximum aperture no smaller than f/3.5 (and f/5.6 normally).