In this area, other systems are better. For example, the Sony SLT A77, since it has a semi-transparent mirror used for traditional PDAF, which also works during video recording. Another system which does this well, is the Nikon 1 mirrorless system. With on-sensor PDAF sensors, it implements AF-C rather well.
Still, Micro Four Thirds cameras get better at this. When the Lumix GH3 was introduced, I found that it did much better than the GH2, probably due to better and faster image processing algorithms. Can the Lumix GH4 improve this further?
To test this, I used pairs of the same lenses on the GH3 and GH4, to compare them head to head:
From front to rear: Lumix G 14mm f/2.5, Lumix G 20mm f/1.7, and Sigma 30mm f/2.8 DN. More about the lenses later in this article.
Here are the results, where you can see the GH3 performance to the left in the video, and the GH4 to the right.
We see that the Lumix GH4 does indeed focus faster during video recording, especially with the Lumix G 14mm f/2.5 and Lumix G 20mm f/1.7 lenses. According to the marketing material, the GH4 contains profiles for the out of focus rendering of all Panasonic lenses. This could be one reason why the GH4 focuses these lenses better, especially the 20mm lens.
With the Sigma 30mm f/2.8 DN lens mounted, though, the GH4 does not consistently do better. In fact, in some scenes, the GH3 focuses the lens faster. This could be due to the GH4 only having the Panasonic lens profiles programmed, not the Sigma lenses. Also, the GH3 has a more mature firmware. The GH4 is still in the 1.0 firmware, and we should expect some further improvements as newer firmware versions become available.
One conclusion is that the GH4 makes the Lumix G 20mm f/1.7 pancake lens more useful for video.
I had both cameras running at 1080p, 50fps. The ISO was set to 200, the base level, and I used the biggest aperture for the most out of focus rendering. I used the "auto everything" AF mode, in which the camera tries to guess which AF method will be most successful, including face detection.
The Lumix G 14mm f/2.5 pancake is the smallest Micro Four Thirds lens from Panasonic. Often, you'll find that it has a poor reputation, however, in my opinion, it is a fine lens. I think the poor reputation comes from the lens being sold cheaply on auction sites, coming from split camera kits. It focuses quickly and virtually inadibly.
The Lumix G 20mm f/1.7 pancake was the very first Panasonic prime lens from Micro Four Thirds, and became an instant classic. Very compact, fast, and very sharp, there is little to complain about with this lens.
One issue, though, is the focus. It has a traditional focus mechanism, in which the whole lens array moves back and forth. This is slow, and noisy. With the combination of non-internal focus design, and a large aperture, needing a precise focus, this lens is widely known for being the slowest autofocus lens in the Micro Four Thirds lineup.
The lens was recently redesigned. However, the updated lens only has a new exterior, not a new optical design. In my test, I found that the new lens focuses just as slowly as the old lens. So there is little need to source the new lens if you already have the old: Functionally they are the same.
The Sigma 30mm f/2.8 DN is one of my favorite lenses: It is very affordable, relatively compact, well performing, and focuses quickly. The only downside is that it is not very fast, for a prime lens. But for a good optical performance at a reasonable price, it is hard to beat.