Hi,
If there is a more appropriate forum for me to pose this Q at, please let me know.
Try "Digital Video Talk". Maybe a mod could move this thread there.
I'd appreciate any advice on this dilemma: I'm shooting (video) under stage lights soon (Northcote Social Club here in Melbourne) and NONE of the standard shutter speeds neutralise banding. Thank god my Sony FX30 has a variable shutter setting - only when I use shutter speed 1/53 - yes, *53*, does the banding disappear. My Sony A6400 - my B-roll camera - does NOT have this option, I'm stuck with standard shutter speeds, NONE of them eliminate the banding. Is there ANY way to get a more precise shutter speed on the A6400?
I have entered a youtube / "how to fix banding" psychosis - EVERY video I find tells me that banding is *always* fixable with one of the standard shutter speeds. This is absolutely not the case.
Yeah, if thay say that it's always fixable with standard nominal shutter speed settings, they don't know what they're talking about. Some light fittings flickers at frequencies that are not multiples of 50.0 or 60.0 Hz.
Mains-related flicker is
almost always fixable with standard nominal shutter speed settings, partly because cameras use different true shutter speeds for video and mechanical shutter or EFCS stills.
For example, shooting stills with mechanical shutter, true shutter speeds follow an EV scale, but video shutter speeds are tweaked to minimise flicker and banding (which are manifestations of the same phenomenon). For example I have a few cameras which at a nominal "1/125" setting use a true shutter speed of 1/128.0 s for stills, and 1/120.0 s for video.
The idea is to make use of this:
The red graph is log2(sinc(x)). The blue graph is log2(1/(π x)).
... to attenuate the fundamental flicker frequency and its harmonics to 7-8EV or more below the average (DC) level of the light output.
This is relatively easy with mains-powered low-frequency fluorescent lights, and conventional single-colour, non-dimable domestic mains-powered LED lights. These lights have relatively high DC output (65%+ of peak output) compared to the output at the fundamental flicker frequency and its harmonics, and the output at high harmonics falls fairly rapidly with frequency.
The problem with some theatre lighting (and some domestic lighting such as colour-changing LED bulbs) is that, at least when operated at low output power, the light output becomes strobe-like, consisting of periodic narrow pulses of light at some fairly constant frequency (low duty cycle Pulse Width Modulation, "PWM"). Sometimes this is done simply for cost reasons, sometimes because of concerns about LED colour shifts at low currents, and sometimes because of difficulties accurately regulating LED currents over a range much beyond 4EV.
In this case, the DC output can be very much less than the peak output over a cycle (perhaps a few percent of peak output), and be smaller than (about half) the amplitude of the fundamental flicker frequency and its first few harmonics.
What all this means is that PWM'ed lights can be a 10x worse (few EV worse if you prefer) problem than "everyday" flickery lighting, such as old magnetically ballasted fluorescent tubes.
I was going crazy with this until I discovered the variable shutter speed option on my FX30 and that magic number - 53 - fixed it - for THAT camera only, though. PAL / NTSC of course makes no difference at all - it's all about shutter speed, from what I have learned so far. My tests have proven this as well.
Yes, it's about the shutter speed, in spite of folk who claim frame rate has something to do with it.
My
guess is that the stage lighting is LED-based, and that when it is heavily dimmed, it doesn't use analogue dimming, but starts using Pulse Width Modulation with a low duty cycle at a frequency close to some multiple of 53Hz. Likely in the hundreds of Hz or low kHz.
Now that you have found one true shutter speed (1/53 s) that works, we can be sure that 2/53 s = 1/26.5 s and 3/53 s = 1/17.7 s, and so on will work, though 1/17.7s is too slow for conventional video. These shutter speeds work because they also give exposure times which are whole number multiples of the period of the flicker.
It is extremely unlikely that the stage lighting is PWM'ed at 53Hz. 53Hz would cause strobing effects obvious to the naked eye. The stage lighting likely runs at at some whole number multiple of 53Hz, likely in a range of about 10*53 Hz to 100*53 Hz.
This gives us a way to guess other true shutter speeds which give exposure times which are whole number multiples of the period of the flicker.
For example, 1/(53*2) = 1/106 s is likely to work, and 1/(53*3) = 1/159 s and and 1/(53*4) = 1/212 s are also quite likely to work.
...
When you write "1/53" do you mean "1/53.0" ? I don't know what resolution "Synchro Scan" has on an FX30.
What frame rate are you using? The reason that I ask is that reducing the frame rate might give you a bit more choice of shutter speed.
My
wild guess is that the stage lighting is LED-based, and that when it is heavily dimmed, it doesn't use analogue dimming, but starts using "Pulse Width Modulation" (PWM) with a low duty cycle (the light is mostly off, and so quite strobey) at a frequency close to some multiple of 53Hz. Likely in the hundreds of Hz.
Does the flicker go away if the lights are turned up to maximum, or close to maximum power?
The smart-alec solution is "buy another FX30" but unfortunately I can't magically make that happen in time for the shoot in a couple of weeks....
Err,
rent another FX30 for the shoot?
Any advice, including other forums to post at, would be appreciated. Thanks....
