The standard MFT sensor has half the dimensions of a full-frame camera's, so a 50mm lens yields a field of view equivalent to a 100mm lens. This makes it tough to get a genuinely wide look at the action.
There's a lot of vintage glass for C-mount cine cameras that would do the trick, but while they cover Super16-size sensors (such as those found on the Blackmagic Pocket Camera and the Digital Bolex) their image circles don't make it to the corners on MFT.
Bear in mind that the Blackmagic Cinema Camera's sensor is slightly smaller than standard MFT. The crop factor is 2.39, rather than 2, meaning that that 50mm lens would have a 120mm-equivalent field of view.
Though the BMCC produces exquisitely filmlike footage, this peculiarity means we get the worst of both worlds when it comes to shooting wide. Old-timey cine glass doesn't cover its sensor, but modern lenses are cropped even more sharply than usual.
It's hard to find MFT prime lenses much wider than 12mm or so (SLR Magic, Rokinon, Olympus), which gives us a 28mm-equivalent field of view. Panasonic makes a 7-14mm zoom, but it's f4 and $1000 and won't work right on the BMCC's passive mount. Rokinon (also branded as Samyang) sells a 7.5mm fisheye, but it's not much faster. The tightest safe bet seems to be the 10mm Rokinon—still 24mm-equivalent on the Blackmagic.
But what if I told you there was a 6mm non-fisheye lens that provided total coverage of the sensor, and is f1.8 to boot? That provides a genuinely ultra-wide viewing angle, equivalent to 14mm on full-frame sensors.
It's the Kowa LM6HC 1" 6mm CCTV lens, found for about $500 or so. It's c-mount, so you'll need an adapter. I had to try three before I found one that permitted the Kowa to focus to infinity. One from a reputable manufacturer such as Metabones might be the best bet.
I'm quite happy with the results! Here are three still frames from the BMCC, shot in RAW 2.5k and output here as 1600×900 high-quality JPEGs.
(Here's a 16MB zip of them output as 2400×1350 24-bit PNGs)
The lens has shortcomings. It's not fisheye, but it is distorted. The bulging glass means there's no easy way to mount filters (it doesn't even have a thread, so the obvious trick of stacking step-up rings won't work). The aperture and focus rings are fiddly. It's obviously a well-engineered gadget, but just as obviously intended for machine vision rather than cinematography.
Here's video, with some other lenses, on the BMCC. You'll know it when you see it. The other lenses are a 12.5mm Fujinon, a 17.5mm Voigtlander, a 35mm Fujian, and a 56mm Helios 44-2.
The question, now, is whether it covers a full-size MFT sensor, too. Could you use it on a GH4? Why yes, you can!
Here are some 6mm videos by others using the (smaller-sensor) Blackmagic Pocket
