I haven't done much with video but in the last few weeks, I've taken over 100 video clips.
While it seems easier to get good quality 1080p video than 16 MP photos, it isn't easy. The camera tries very hard but it has to struggle a bit to handle auto focus, exposure, and more, even with an f/2.8 lens.
I'm not a film maker, and I don't expect to be. However, I had a short conversation with someone who is, and he recommended the Metabones SpeedBooster and Sigma 18-35mm f/1.8 for the Panasonic GH4, a combination he uses.
It's not cheap, and it's not weather-resistant, which could be a problem, as most of my equipment already is. I must always be aware of the difference because it would be an expensive disaster if I would forget.
I've been to slow to embrace Sigma lenses. When I sold equipment, they were a remote third party with poor quality. Even recently, with Four-Thirds lenses like the 30mm f/1.4, I wasn't quite comfortable. The lens wasn't calibrated properly (for anyone, apparently) and it had a yellow cast.
Recently, they've been coming up with some good lenses. The ART series is getting good reviews, although they started by re-labeling their micro Four-Thirds lenses with "ART", to make them seem better than they were. I'm still not sure the 19mm, 30mm, and 60mm f/2.8 lenses are worth what they charge--and around $200 each, they're not horribly expensive.
The 18-35mm f/1.8 made a huge change. I was shocked. I've been using the Olympus 14-35mm f/2.0 for a while, and it is an impressive lens. If Sigma could do more for a third of the money, I would be surprised, weather-sealed or not.
Still, it comes in a limited number of mounts, and that wouldn't include Four-Thirds or micro Four-Thirds, naturally. Then, Metabones arrived with their adapters and SpeedBooster focal reducers (reverse teleconverters).
Can you turn an existing lens into a faster (lower aperture number) lens just by adding an adapter? Apparently, you can, and the other effect is that you get a wider focal length. According to the chart, you reduce the focal length multiplier from 2.0x to 1.42x for a typical micro Four-Thirds camera body. It boosts the lens' effective aperture by one f-stop. Since I haven't found any photos above ISO 3200 to be useful, it would be interesting to see how far into the dark I could shoot and still have useful photos.
I've considered using lenses with an aperture of f/0.95 but I'm not sure if those Voigtländer lenses are what I want. They certainly cost a bit, for an experiment, and using a single focal length lens is not typical of my style, having had so many great Olympus zoom lenses.
If the Sigma 18-35mm turns into 36-70mm effective 135 Format equivalent focal length normally, 25.56-49.7mm with the focal reducer seems close to the lens' target for APS-C, 27-52.5mm. Considering how difficult it is to get wide angles with the Four-Thirds sensor size, any focal reducer is appreciated.
I'm also considering the Tamron 15-30mm f/2.8 lens, especially since it is weather-sealed/moisture-resistant. Of course, I'm also thinking about a couple of Nikon bodies--the D750 and the D7200 to use these lenses natively, responsively.
Update 2015.04.13: Considering the difficulty of shooting in low light, the D750 is look better and better. DxOMark gave much better results for the sports ISO number than the D7200 by more than 2x and naturally, almost 4x over the GH4 or E-M1. While I consider these numbers to be a guide, something that the company uses for their own software, they are relatively useful.
Regardless, I wouldn't just use the lenses on one system or the other but I'm fairly sure now that I don't want to make films--only sports videos.
Update 2015.04.18: Well, this has changed dramatically in just a few days. I bought a Pentax K-50 kit. The Sigma 18-35mm f/1.8 is available for Pentax but there is no Metabones SpeedBooster adapter. I can use the Voigtländer adapter but without the lovely focal reducer.
Update 2016.01.04: Well, I've had the Sigma 18-35mm f/1.8 lens since July 5th when I also bought a Nikon D7200. I went to buy the SpeedBooster and they changed the products. Since I work with Panasonic and Olympus bodies, this has become a big deal. I could buy the Super35-style 0.64x version for the GH4 and 4K video or the 0.71x for all bodies. I'm not a film maker. I make skate park videos, as well as taking a lot of photos. I also recently bought the Tokina 11-16mm f/2.8 for ultra wide angles, and it could be interesting, if I was making films. I might try some environmental-style videos, as I have done at an aquarium or with wind turbines.