As Waddizzle stated, it's almost certainly "light flicker".
flaviamash: "I tried shooting at different speeds ranging from 1/100 to 1/800 and ISO rates but I dont understand the logic"
Long technical description of theater lighting "flicker". Please move on if you are a technophobe. 🙂
When we mention "light flicker" that does not necessarily mean flicker that is visible to the human eye.
In ancient times, stage lighting used tungsten bulbs controlled by rheostats or variable auto transformers. As brightness was changed for stage requirements, those lamps produced light that was essentially continuous, always on, but at variable brightness.
Modern theaters, and most older theaters now use LED lighting. LEDs are optically very fast devices, the light output can be switched on and off in microseconds at full intensity. That makes it most efficient to control the average light intensity by rapidly switching the LEDs on and off (flickering) at frequencies that are not apparent to humans, but can and will be recorded by cameras with short shutter duration settings.
A 5D Mk-III has a flash sync speed of 1/200 sec. That means that 1/200 sec is the highest speed setting that the whole image sensor area is fully exposed at the same time. The time "window" that the shutter is open is 5 mS long. That works fine for a flash exposure which is typically only 1 millisecond or shorter.
However, virtually all modern theater lighting systems using LED dimmers use some form of PWM (Pulse Width Modulation) to control lighting intensity. Depending on the type of controller, the LEDs may be flashing at a rate of 50, 60, 100 or 120 Hz (depending on country and if the controller is "half-wave" or "full wave". Additionally, there are now more modern "High frequency" controllers in use that send a variable number of very short duration current pulses to the LEDs at rates of up to 2000 to 5000 per second which vary the average LED intensity by modulating the number of very short pulses, not their width.
The net result of all this light "pulsing" is that at shutter speeds higher than about 1/30 sec, there is a strong possibility of overall (full-frame) exposure variations from shot to shot depending on when the shutter triggers in relation to the instantaneous phase of the PWM controller. AND, for exposures faster than 1/30 sec intensity banding is possible, and for exposures above 1/200 sec, multiple close-banding is possible.
Exactly what happens depends on the particular type of lighting controller system the theater is using so other than using very slow shutter speeds, it's imperative to take a series of test shots if possible during a rehearsal.
Obviously, it would be great to be able to use high shutter speeds to stop motion, but doing so introduces a risk factor for lighting-caused artifacts. Setting the camera for a burst of 10 or 12 shots at high shutter speed may (again depending on the type of lighting controller in use) increase the likelihood of getting an acceptable shot.
The probability of banding happening is also affected by the actual setting of the light intensity, so may change during the theater production. At high or near full brightness, the LEDs are on for all or a high percentage of each time cycle. At mid to low settings the LEDs are off for more time per cycle than they are on so banding is more likely.
Hope this helps you "understand the logic".
Thanks for making it to the end. Please pickup and answer your 10-question quiz and bring it to Tuesday's class.
