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HMI flicker / banding on flicker-free ballast?


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#1 Jay Young

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Posted 02 May 2017 - 07:39 AM

Shooting with an Arri 1.2K HMI Fresnel, looking at the scope the blue channel was waving at me.  

 

We were shooting 45-degree shutter angle, at 24 frames per second. 

 

Gaffer put the HMI on "flicker-free" but the banding still continued.   We tested different frame rates and shutter angles along with setting the ballast at 60/50Hz and "Flicker-free", but it still banded. 

 

I ended up shooting at 172.8-degree shutter for that particular shot because I couldn't get the lamp to cooperate with the camera settings. 

 

 

Anyone else experienced this?  I assume it was a wonky ballast as I have used the 1.2K Par with no issues on the same camera settings. And I can't believe the Fresnel would be all that different than the Par in the ballast department. 


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#2 Edward Lawrence Conley III

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Posted 02 May 2017 - 01:03 PM

Could be an old globe too.

 

 

Should have a spare on set- rentals are sent out with a spare globe.


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#3 Jay Young

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Posted 03 May 2017 - 08:40 AM

Could be an old globe too.

 

 

Should have a spare on set- rentals are sent out with a spare globe.

 

Lets just say that we had some green grips/electric - and when I went to the truck after wrap I discovered that all of the things I had ordered were in fact on the truck, including spare globes... AND TAPE.... they'll learn some day. 


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#4 George Ebersole

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Posted 03 May 2017 - 09:03 AM

What kind of camera were you using?


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#5 Jay Young

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Posted 03 May 2017 - 09:41 AM

What kind of camera were you using?

 

This particular shoot a Blackmagic Ursa 4.6k with the new operating system.  The black shading calibration tends to affect the blue channel far more than the others, which is something I didn't really have to deal with on the 4K sensor, which is my preferred chip. 

 

The only reason I mentioned the blue channel is that was being affected most by the HMI flicker.  It would calm down after re-calibration on the black shading, but still was visible.  I ended up just shooting those sequences at 180-degree shutter, which solved the issue.  


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#6 George Ebersole

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Posted 03 May 2017 - 06:44 PM

When you mentioned shutter angle I thought you were shooting film.  There's an old trick that sometimes could clear up flicker by taking advantage of the cycle rate on power feed you were using.  It probably won't work with digital.


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#7 Robin R Probyn

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Posted 03 May 2017 - 07:17 PM

Works the same on digital doesnt it.. I can get rid of flicker using shutter with digital camera.s.. or am I missing something.. ?


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#8 Satsuki Murashige

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Posted 03 May 2017 - 10:35 PM

Is 1/192 a flicker-free shutter speed, though?
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#9 Robin R Probyn

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Posted 03 May 2017 - 11:36 PM

Dont know .. was just referring to Georges post that seemed to imply using shutter to get rid of flicker was different between a film camera and a digital one.. or actually wouldn't work with digital more precisely   as far as i know they function the same way.. Ive got rid of flicker with both types using shutter the same way.. but i could be missing something?.. 


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#10 Jay Young

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Posted 04 May 2017 - 10:00 AM

Well I learned some things today.  Mostly that 23.98 is just simply not HMI safe.  One can minimize it, but 24 fps is not the same as 23.98 when it comes to power and flicker calculations. 

 

I'll just shoot at 24 from now on and let post figure it out. I don't have time on set to solve these issues trying to keep post-production happy and efficient. 


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#11 AJ Young

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Posted 04 May 2017 - 03:18 PM

Were you using an electric or magnetic ballast?

 

I've never had issues at 23.98 on an electric ballast.

 

EDIT: Was it an ARRI brand ballast?


Edited by AJ Young, 04 May 2017 - 03:18 PM.

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#12 Jay Young

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Posted 16 May 2017 - 12:38 PM

Were you using an electric or magnetic ballast?

 

I've never had issues at 23.98 on an electric ballast.

 

EDIT: Was it an ARRI brand ballast?

 

I see my last post never made it for some reason...

 

The ballast was electronic, and ARRI brand. 

 

I solved the issue this time round by renting 10k's..... I have nothing but problems out of HMI's - personally. 

Although the Joker 200/400 is nice. 


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#13 Phil Rhodes

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Posted 16 May 2017 - 01:03 PM

Some HMIs certainly do this. Even Arri ballasts, alarmingly. One would have thought that things which are that expensive would be less, er, crap, but that's life.

 

Flicker free is a relative term, it seems.

 

P


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#14 Matthew Kane

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Posted 16 May 2017 - 01:27 PM

What was your power supply? By any chance, was it a high speed ballast? I'd be curious if the problem persisted at 1000hz. Was the flicker visible in the footage, or just on the waveform? Also, did any other fixtures exhibit the same behavior?

 

I've never had any differing performance between 24fps and 23.98, even with magnetic ballasts, old globes, etc.

 

If there was a long run of stingers to the ballast, line loss might have been the culprit. Or perhaps an appliance or piece of equipment with a ground fault was on the same circuit. Faulty mains wiring or malfunction at the generator can cause headaches too.

 

Also--maybe the bulb was not fully seated? I'd love to know if you figure it out--always good to learn from other's experience rather than first hand. If the fixture was marked NFG and sent back, maybe the rental house has an idea.


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#15 Jay Young

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Posted 16 May 2017 - 01:57 PM

What was your power supply? By any chance, was it a high speed ballast? I'd be curious if the problem persisted at 1000hz. Was the flicker visible in the footage, or just on the waveform? Also, did any other fixtures exhibit the same behavior?

 

I've never had any differing performance between 24fps and 23.98, even with magnetic ballasts, old globes, etc.

 

If there was a long run of stingers to the ballast, line loss might have been the culprit. Or perhaps an appliance or piece of equipment with a ground fault was on the same circuit. Faulty mains wiring or malfunction at the generator can cause headaches too.

 

Also--maybe the bulb was not fully seated? I'd love to know if you figure it out--always good to learn from other's experience rather than first hand. If the fixture was marked NFG and sent back, maybe the rental house has an idea.

 

Yes, there was visible banding at normal speed, rolling up.  It was being powered off mains, and I am not positive there wasn't something else on that circuit.  Who knows.  I did not solve the issue with the fixture, but rather opened the shutter to 180-degrees which allowed us to continue. 


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#16 Phil Rhodes

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Posted 16 May 2017 - 02:39 PM

This winds me up.

 

Movie industry metal halide lighting gear is very, very, very expensive.

 

I don't care if the mains was slightly flaky.

 

Those things are way, way too expensive to have these problems without warning you, regardless of the circumstances.

 

P


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#17 Matthew Kane

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Posted 16 May 2017 - 05:01 PM

Shame there's no way to know for sure. It'd be interesting to troubleshoot, but usually when I have these issues, there's no time for anything but swapping in a new unit or fixing it in camera like you did. It could even have been a damaged head cable I suppose.

 

@phil, modern HMI gear can do amazing things, but not even a Ferrari works right if an important part is damaged or you're working outside the operating specs.


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#18 Satsuki Murashige

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Posted 16 May 2017 - 05:17 PM

Well I learned some things today.  Mostly that 23.98 is just simply not HMI safe.  One can minimize it, but 24 fps is not the same as 23.98 when it comes to power and flicker calculations. 
 
I'll just shoot at 24 from now on and let post figure it out. I don't have time on set to solve these issues trying to keep post-production happy and efficient. 


Hi Jay,

Since it is ultimately shutter speed and not shutter angle that is the issue with regard to flicker-free camera settings, you could also shoot with the camera in shutter speed mode and still get true 1/48 sec @ 23.98p. Shooting with a 180 degree shutter angle* @ 23.98 results in a true shutter speed of 1/47.952, which is not flicker-free.

*Remember, 'shutter angle' is simulated in cameras with non-mechanical shutters.
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#19 Satsuki Murashige

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Posted 16 May 2017 - 05:18 PM

Art Adams had a really good write up about this issue on his ProVideoCoalition blog a few years ago. I'll see if I can find it.

EDIT. Here it is: https://www.provideo...lickering-hmis/
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#20 Jay Young

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Posted 16 May 2017 - 05:23 PM

Art Adams had a really good write up about this issue on his ProVideoCoalition blog a few years ago. I'll see if I can find it.

EDIT. Here it is: https://www.provideo...lickering-hmis/

 

 

Yes. I read that a few days ago. Then I told the sound recordest we were switching to true 24.  All is well! 


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