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Help! Slow motion flicker issue


nc1983
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Hey guys, 

Desperately need your help.

Tried researching and I'm pretty sure I know what's causing it, I just don't know how to fix it. Whenever I shoot slow motion (120fps) in low light I get a TERRIBLE shutter flicker. I know it may be the lights themselves but is there a solve for shooting in these conditions?

I tried Phillip Blooms tip of overlaying the footage with the same clip, and move it one frame forward and reduce opacity to 50% but I have to move mine 3 frames and that creates too much blur.

Here is an example of my footage and issue. SHOT @: 120fps 1/250 shutter:

SO grateful to any insight on this...

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The effect is being caused by the frequency of the lights not matching up with the shutter speed of your cam. You need to adjust the shutter speed so that its timing matches up with the lights frequency. In the US our lights operate at 60hz and output 120 pulses per second. Your shutter speed of 250th is catching the light at different points in its cycle. If you imagine 2 waves that are slightly out of synch with each other. Which is why lining up the footage in post can work to mask the effect because you are stacking the waves over each other to cancel out the difference. Your shutter speed needs to be divisible by 60, so you may need to lower your fps so you can use slower shutter speeds. 120th and 100th of a second usually work for me when I use vfr on my gh4. A 240th of a second shutter -should- work since it's divisible by 60 but the gh4 at least does not have that option.

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Your shutter speed needs to synchronize with the power frequency of the lights. 

In the U.S. the alternating current (AC) frequency is 60 hz. It's 50 in Europe. 

Anyway, there's a ton of info out there on the Google about this. 

It's certainly research you need to know if you're gonna be a shooter. 

Alternatively (excuse the pun) you could just shoot in daylight. Those photon waves move so fast a camera shutter speed doesn't matter.  Ask Kelvin about that. 

Seriously though, cinema is a technical craft as well as an artistic one.  So, rtfm ;-) and good luck. 

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Those exterior light sources are ballasted, so their output frequency could anything (not necessarily 60hz in US nor 50hz elsewhere).

 

Don't know what you can do with your existing flickering footage, other than using some deflicker plugin:

 

As others have suggested when shooting again with such lighting, try changing the shutter speed:

 

Also, if possible, try dialing your frame rate incrementally up and or down.

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I am facing the same issue. We shot a music video, but the shutter speed was badly set. Phil Blooms tutorial helped me a lot: 

However, this technique creates blur, so I have to replace each clip with an AE composition, mask around the subjects, duplicate the wrong areas and push it by a frame. Not perfect, but you can see the difference.

 

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Yes I shot this in Portugal (night of the Euro win). This is all so helpful guys. I'm definitely still learning and being novice, I didn't want to break from the rule I learned of setting your shutter speed 2x your frame rate. Clearly I need to for this. I'll run some tests in Portugal tonight and see what works out best. I will also switch from 120fps to 100fps. Luckily these shots are going to be quickly cut so most people I've talked to said they didn't even notice until I pointed it out. Really appreciate the help!

 

 

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4 hours ago, Mattias Burling said:

Just to clarify, it's the frame rate that's the problem, not shutter speed.

In Europe we shoot 50 and 100fps for slow motion. 60 and 120fps is for Canada, USA and Japan.

So, switch to 50 or 100fps when close to electrical lights and the problem is solved.

Mattias, no hard feelings but this time I think you are wrong. I shot 60p with 1/100 shutter speed (in Europe) to be conformed to 25p so i could squezze a little bit more than 50% slowmo and didn't have any flicker. I actualy planed to shoot 50p but then did some research and found this online flicker free guide/calculator on RED webpage

http://www.red.com/tools/flicker-free-video

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Always a good idea to record a clip, then play back from camera to check (often flicker does not reveal itself until conformed to normal speed playback).

Different light fixtures can also have varying frequencies sometimes (Neon,LED etc) - so the quickest method I find is to record a little test clip, rather than rely 100% on the mathematically 'correct' solution. The longer test clip you can do - the better, as that can also reveal phasing issues (slower speed banding).

In this case, the flicker calculator mentioned above would be a sensible start - then confirm that with a clip test.

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