In 3D the cameras need to be much closer to the front of the stage which does cause sightline issues if you have an audience in. For Swan Lake there were five 3D cameras including two dollies and one on a crane, plus two 2D cameras all set up across the front of the stage. This entailed taking out quite a few rows of seats. I'd always assumed all 3D filming used two lenses next to each other to get a 3D effect but the set up for Swan Lake was a mirror rig which has cameras set at 90 degrees and uses a mirror.
What was even more impressive about this was the OB truck parked outside. Apparently it spends most of its time parked outside football grounds doing live 3D broadcasts of Premiere league matches. It expands outwards like something from Transformers.
Inside was more like Close Encounters with us all watching the action wearing 3D glasses.
This is where the director (Ross McGibbon) and the producers sat. To the left was sound and to the right an individual operator for each camera.
As 3D is so new lessons are being learnt all the time. Apparently 3D can look gloomy compared to shooting in 2D and fast movement is hard to pick up. Also the effect can be distorting with a big drop off between foreground and background (critics of the Mariinsky's 3D film mention this).