In shooting 3D you naturally have two cameras and two lenses: albeit, there are some weird and wonderful single lens/single sensor contraptions out there. This usually means there are any number of ways each camera lens combination could be producing images with minor are major signal level differences. If these level differences are not dealt with on-set or in post-production, they could lead the visually objectionable artefacts in 3D portrayal.
When shooting with a mirror rig, there is a about a stop difference in exposure levels between the cameras due to the optical losses in the 45 degree half-silvered surface. .It is also necessary to be rigorous about checking that the internal camera filters and any external filters, shutter settings and iris combinations produce matched exposure levels. If this is not done as passed down to post production, then there will be a loss of (bit) resolution on the darker image. Any compensation down-stream with lift and gain results in a loss of resolution (quantisation) and an increase of noise level in the boosted eye channel. So it is best, where possible, to do it right on-set.
Measuring exposure levels is best done with waveform monitor and vectorscope displays. The waveform monitor is an easy concept to grasp: a real-time graph showing the signal levels. At the bottom are the blacks and at the top the whites. But to get the ‘eyes’ balanced using a conventional waveform monitor would be tricky to say the least. A better representation is to overlay both traces for the left and right channels. If these were in the same colour, you could not see which was which.
However, there are now specialised 3D dual –stream devices with waveform displays now available. One technique is to show the left and right eye traces in Red/Cyan anaglyph colours. This means that if levels are matched the colours combine to white. Whereas if it were the other way round and the right eye luminance were lower than the left you would see red coloured waveforms over the top in Cyan (Bluey/Green).
The same Red/Cyan trick can also be done other types of monitoring display such as the vectorscope to check colour matching. The RGB parade display is appropriate really only if there is a CCU (Camera Control Unit) associated for each camera with RGB adjustments available to the operator for individual black level trim and gain trim level settings. One further advantage of these overlaid displays with standard Red/Cyan anaglyph glasses; you can see each channel by blinking alternate eyes. But don’t try to view 3D this way: you could wear out your eye-lids!.
The Cel-scope3D stereoscopic analyser includes an ability to show waveform, vector and various parade modes together with histograms in combined displays colour coded in Red/Cyan for easy level balance and matching.