First of all, let's note that there is disappointing news about the E-M5 Mark II video: The sensor is cropped slightly, making your lenses less wide than you expect. Here is an illustration of the sensor area used for video:
This corresponds to an additional crop factor of 1.16. Or in other words, the Olympus 12-40mm f/2.8 becomes 14-46mm when using the video mode. So the effect is not very dramatic.
This crop during video is not uncommon, by the way. The Nikon D7200 can only do 1080p video at 60fps with a 1.3x crop of the sensor. And that is Nikon's premier DX DSLR. While we wait for the D400.
To compare the two cameras, I put the Sigma 60mm f/2.8 DN on the Lumix GH4, and the Olympus 45mm f/1.8 in the E-M5 Mark II. That way, the lenses correspond to about the same field of view. I set both to f/4, and used ISO 200:
I think we see that the Lumix GH4 has slightly better resolution, but the difference is rather subtle.
Video image stabilization
The Olympus OM-D E-M5 II also provides a very functional in-body image stabilization feature, which works great during video recording. Here is a demonstration of how it works.
By removing the lens, it is possible to look straight into the sensor, to see how it moves during video recording. To video record the sensor, inside the lens mount, I put the Lumix GH4 with the Samyang 7.5mm f/3.5 fisheye lens pointing straight into the E-M5.
For an even lightning, I put some white paper around the Samyang fisheye lens. The lens was set to the closest focus distance, and f/8 for sufficient depth of focus. When moving the camera around, one can see how the sensor also moves, to stabilize the video:
And to compare it with the Panasonic lens based image stabilization, I have recorded handheld footage using both a Panasonic based system and an Olympus based system. Here is the setup:
And here is the result:
As you see, even when using a long lens, the Olympus OM-D E-M5 II is capable of stabilizing the video very well. The Lumix GH4 with the Lumix X 35-100mm f/2.8 does not do nearly as well.
Autofocus during video
Continuous autofocus for moving subjects is the achilles heel of mirrorless cameras. So far, Nikon 1 cameras solved this already from the first generation by using on chip PDAF sensors, see my test here. Also, the latest generation of Sony cameras gave gotten up to a good level, e.g., the Sony a6000.
But so far, Micro Four Thirds cameras do not use on-sensor PDAF for autofocus during video recording. Rather, they rely on image processing to assess which way to turn the focus. Most of the time, this works pretty ok with recent cameras.
Here is a test comparison with the Lumix GH4 and the Olympus OM-D E-M5 II. First, I used a pair of Lumix G 20mm f/1.7 pancake lenses (the same lens on both cameras), which is known for being slow to focus. In the second half of the video, I used a pair of Sigma 30mm f/2.8 DN lenses.
Both cameras were used in 1080p, 60fps. Here are the results:
As you see, the Olympus camera keeps up the focus quite well during the video, and some times better than the Lumix GH4. Even if the GH4 probably has more processing power, and boasts about the DFD (depth from defocus) technology, it does not consistently beat the E-M5 II.
Example low light video
These videos are not from the same gig, but the lightning is similar, and very dim in both cases.
Lumix GH4 with Lumix G 20mm f/1.7 at ISO 6400, f/1.7, 1/60s:
Olympus OM-D E-M5 II with Lumix G 20mm f/1.7 at ISO 6400, f/1.7, 1/60s:
I think the video stabilization implemented on the Olympus OM-D E-M5 II is truly amazing, and it works with all lenses, even with adapted manual lenses, provided that you key in the focal length used.
On the other hand, the video resolution is not quite up to that of the Lumix GH4, and the autofocus could still be better. But with the E-M5 II, I think Olympus has brought video to a new level.