The main problem with the H.264 codec in the OM-D is the ridiculously low average bitrate, the manual says 20Mbit/s but it sort of averages out at 17Mbit at best and sometimes considerably lower. It's a VBR codec (Variable Bit Rate) which means detail level in the video stream prior to encoding is the main trigger for bitrate distribution. More details in a scene will trigger the codec to run at higher bitrates, but at a certain point too much detail will cause the encoder to more or less 'pack its belongings and go home for the day'. See image below for the worst case scenario, ruined shot!
- Picture Mode: Muted
- Contrast -2
- Sharpness -2
- Saturation 0
- Gradation: Normal
Bump up your ISO to 640 as a kind of 'base' ISO for all your work, if you don't mind a little bit of fine filmic grain in your footage. It seems to trigger the codec, if ever so slightly, to produce higher bitrates and improve upon IQ and stability. This is my humble opinion. Using a variable ND filter in front of your lens will be a necessity if using fast lenses in daylight conditions.
Avoid fast pans and slow pans. In fact avoid panning at all cost, especially with detailed scenes. Shoot at large apertures for shallow DOF and preferably with manual lenses like I said before. The E-M5 can produce some very stunning footage if used the right way. Ironically the codec produces some very fine grain and detail in static scenes but it appears to deteriorate very easily if any stress is applied on the codec.
After all that being said, I just learned that Olympus OM-D (E-M5) cameras were used along with ALEXA's and Canon EOS 5D's during the principal photography of the new upcoming Mad Max 'Fury Road' movie, see related article here. So it has a place in cinematography despite its shortcomings.