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Blue channel pulsing issue

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#1 Frank Barrera

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Posted 23 March 2018 - 01:01 AM

Hi.

 

I am three weeks into a 4 week show and today our dailies colorist alerted us that we suddenly are experiencing a "pulsing" in the blue channel in footage we shot early this morning. Something akin to an out of sync HMI when shooting film. But we are not shooting film. We are shooting with two Sony F55s. The issue is with both cameras so we suspect the lights.

 

Our lights are Arri Sky Panels, M8's, M18's and M40's. 

 

The "pulsing" can not be seen by the human eye during playback but can be seen in the RGB Parade.

 

The footage is usable but there is a concern about potential red flags for QC.

 

Our schedule is extremely tight. We have zero time to do any sort of testing of our lights. 

 

Does anyone have any insight and or experience with perhaps the Sky Panels resulting in blue channel problems?

 

thanks

 

f


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#2 Phil Rhodes

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Posted 23 March 2018 - 06:07 AM

An F55 can have a problem with an HMI the same way film can.

 

Silly questions first:

 

Are you shooting conventional frame rates and shutter angles?

 

Do you have the HMI ballasts in a suitable mode for what you're doing? Have you tried other modes, or adjusting frequency where it's adjustable? A lot of M-series lights will have the modern adjustable-frequency ballasts. If you do this be very careful that you don't introduce more problems.

 

Are both cameras running exactly the same firmware and with precisely identical configuration in all respects? Have you used a settings-transfer feature to move the complete settings stack from one camera to another?

 

Are you using any lens accessories such as a rain deflector, polariser-based variable ND, any other type of polarisation, or ND grads?

 

Are you using lenses with automatic iris or image stabilisation features?

 

Is there anything between the camera and the waveform? Is the problem visible on all outputs from the camera, both with and without LUTs?

 

Is there any mechanical fault with the ND filters in the cameras?

 

Are you using any advanced features of the Skypanels such as flicker effects, and have these effects been properly disabled?

 

Are you using DMX control on any of the lights, including HMI ballasts?

 

Have you tried moving the dimmer controls on the ballasts to ensure a dirty pot isn't causing the issue?

 

Is there any other light source leaking into the setup that may not be continuous, either at high frequency (old fluorescents, etc) or simply sunlight with a rotating fan in front of it?

 

Is the mains clean? Are any generators in good working order, not overloaded, etc?

 

Try using a cellphone to detect flicker. Hold it close to the light so that it winds down its shutter timing to a minimum, then watch for banding in its rolling shutter. This can be a highly sensitive test for flicker.

 

My guess: lights, probably the HMIs, but really you need to find a way to at least do the work to isolate this to a single bit of the lighting package, then replace or simply stop using it.

 

Skypanels are specced to shoot up to 25,000fps without flicker but I have not tried it and I am not, overall, that convinced by Skypanel as a product.

 

Also, when you find it, report back here so it goes into the collective consciousness.

 

P


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#3 Frank Barrera

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Posted 23 March 2018 - 08:28 AM

Phil,

 

You are exactly who I was hoping would respond. I will run down all of these questions today. But I am hoping the "other light sources leaking" is the answer. We did have some practical lights on in shot. We were moving so fast that in addition to our lighting we turned a couple of the practicals on also and since we didn't see any flicker (which we weren't even looking for) we just went ahead. As soon as I get to set today I will take a look at those bulbs.

 

Stay tuned and thank you very much.

 

F


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#4 Bruce Greene

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Posted 23 March 2018 - 10:36 AM

I'm often shooting 24fps in Europe and I see this quite often from practical lamps, signs, and TVs.  Changing the shutter angle often helps, but sometimes can not completely fix the issue.  It's not a bad idea to throw the waveform up on your monitor just to look for pulsing like this, but sometimes there's nothing that can be done to fix it.

 

Often, in places like night clubs these days there are many TVs and they are not all at the same frame rate or frequency.  (live TV vs. playback from a computer) and there is nothing that can be done except to replace the flickering images in post fx, but the light spill still flickers...


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#5 Frank Barrera

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Posted 23 March 2018 - 01:26 PM

No other footage from yesterday has the flicker issue so we believe it had to be the over head practicals. I'd love to get back into that location and confirm that they were some old compact florescents or something but that location is long gone for us.

Of course I have seen flicker live at video village when shooting TV's and computer screens etc. But this is the first time I've had flicker only in RGB parade and not visible to the eye. Obviously this has had to have happened before on other shoots but no one in post caught it or bothered to tell me about it.

Thanks for the help fellas. Disaster averted.

F
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#6 Adrian Sierkowski

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Posted 23 March 2018 - 04:29 PM

Had this happen to myself as well recently at a location where they had retrofitted in LEDs into the chandeliers. We mucked with the shutter angle a bit to get rid of it; it was imperceptible to eye and only barely there on the waveform (you had to really look for it, without anything else moving in frame). 


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#7 Robin R Probyn

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Posted 23 March 2018 - 06:50 PM

No other footage from yesterday has the flicker issue so we believe it had to be the over head practicals. I'd love to get back into that location and confirm that they were some old compact florescents or something but that location is long gone for us.

Of course I have seen flicker live at video village when shooting TV's and computer screens etc. But this is the first time I've had flicker only in RGB parade and not visible to the eye. Obviously this has had to have happened before on other shoots but no one in post caught it or bothered to tell me about it.

Thanks for the help fellas. Disaster averted.

F

 

 

From what I hear there is also some pretty sophisticated, but easy and quick, post software now to correct flicker.. but yes best to avoid in the first place of course..


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#8 Frank Barrera

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Posted 24 March 2018 - 11:36 AM

Ok. So, we were able to get back into that location and look at the bulbs in the over head practicals. The culprit was an off the shelf household LED bulb. It was directly over our actors' heads. 

 

Lesson learned: LEDs cannot be trusted. But on low budget productions and in the real world its a challenge to avoid them. 

 

f


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#9 Mark Dunn

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Posted 24 March 2018 - 12:50 PM

So is a BB gun now essential grip equipment :ph34r:?

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#10 Adrian Sierkowski

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Posted 24 March 2018 - 12:51 PM

Try a 216 degree shutter angle (and 178.2) ;) easier than transpo of a bb gun, and much safer in today's US.


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#11 Phil Rhodes

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Posted 24 March 2018 - 01:05 PM

Called it. But then again I called about 20 things, so perhaps I'm not the expert I thought I was.

 

That'll work on some of 'em, which use capacitive droppers or long series strings of LEDs and therefore operate at the same frequency as the mains. Others will use switch-mode electronic drivers which may also flicker, but at any of a lot of frequencies.

 

P


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