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Wierd strobing/flickering on PAL 7D footage

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Mieke Vlaming
Wierd strobing/flickering on PAL 7D footage
on Oct 23, 2011 at 6:45:02 pm

I have recently received footage that was shot at 25fps, 1920x1080, on the Canon 7D under fluorescent lighting, and there is a very strange flicker/strobing that I assume to be caused by a discrepancy with the shutter speed.

The problem is that the colour seems to be shifting from blue to orange and I am very concerned about how this will affect the colour grading process. Does anyone know of a plugin or filter for FCP 7 that could help with this problem, or of anything that I could do to the footage that will lessen this effect?
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Neil Patience
Re: Wierd strobing/flickering on PAL 7D footage
on Oct 23, 2011 at 10:01:01 pm

I suspect you are probably right. The "beating" effect between shutter and light flicker is likely to be the cause.
I am pretty sure that florescent lights flicker at nominally twice mains frequency however the rate is pretty unstable, especially if the tubes are old.
I think its very often a problem shooting under that kind of light for that reason.

You could try the flicker filter (under video) as one possible solution. I know there is a Nattress G deflicker too (Big box of tricks set)
These might offer some assistance if the blue/orange colour change you are seeing is very fast - lighting flicker kind of rate.

If it is more of a drift in colour balance then that's going to be hard to fix with a plug-in.

best wishes
Neil
http://www.patience.tv


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Rafael Amador
Re: Wierd strobing/flickering on PAL 7D footage
on Oct 24, 2011 at 1:26:09 am

[Neil Patience] "You could try the flicker filter (under video) as one possible solution. I know there is a Nattress G deflicker too (Big box of tricks set)"
I don't think those filters will work in this case.
They work for "in picture flicker" (an object or area that flicks) but not for a frame to frame flicker.

You need something to be adjusted up to the flicker frequency and the shift in luma and color.
Sapphire has one to do so, but they are expensive plugins.
I know that can be done in AE using "expressions" and probably could be done in Motion using "behaves" but really I can't help much with those application.
rafael

http://www.nagavideo.com


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Neil Patience
Re: Wierd strobing/flickering on PAL 7D footage
on Oct 24, 2011 at 9:09:37 pm

Hi Rafael

[Rafael Amador] "They work for "in picture flicker" (an object or area that flicks) but not for a frame to frame flicker."

You could be right, Mieke would have to give it a try, however fluorescent lights flicker at around twice mains frequency so in the UK that is 100htz.

That becomes "field" rate flicker rather than frame rate so it wont be frame to frame flickering but in theory about twice per frame.
So that is why I thought it might just have some effect - it is a little hit and miss though.
Maybe Mieke will let us know ?

best wishes
Neil
http://www.patience.tv


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Mieke Vlaming
Re: Wierd strobing/flickering on PAL 7D footage
on Oct 25, 2011 at 7:19:03 am

I think Raphael has the best idea of what the problem is. Sorry, I realize that I probably didn't explain properly. It looks like bands of colour (Blue and orange and about four per frame) moving/scrolling up the screen. They aren't very noticeable when played back in real time, but they become very obvious when I increase the speed or scrub through the footage.

I am an editor so don't know a lot about camera and lighting anomalies, but the fluorescent lighting is the only thing that I could think of that could cause something like this. Initially I thought that the footage had become corrupted when I transcoded from H.264 to ProRes 422 HQ, but the problem exists in the raw footage too. So I can only think that it was shot in camera.

I realize that there probably isn't a plugin (or at least an affordable one) that could fix a problem like this, but I figured that it was worth a try.


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Rafael Amador
Re: Wierd strobing/flickering on PAL 7D footage
on Oct 25, 2011 at 5:09:02 pm

[Mieke Vlaming] "I think Raphael has the best idea of what the problem is. Sorry, I realize that I probably didn't explain properly. It looks like bands of colour (Blue and orange and about four per frame) moving/scrolling up the screen. They aren't very noticeable when played back in real time, but they become very obvious when I increase the speed or scrub through the footage. "
Up to this, then Neil is Right.
The fluorescent flickering is an ld problem but CMOS (as the DSLRs have) increase the problem.
With CCDs cameras you would have differences from frame to frame, or from field to field:
- CCDs "locks" before making the vertical scanning. All the lines belong to the same moment in time.
The full frame (or field) suffer the flicker or not.
- CMOS keeps active while they are being scanned. Each line belong to a different moment in time (this what causes the infamous rolling-shutter), so certain lines will suffer the flicker, certain do not.. This is why you see different "bands" in the same frame (or field).
And this really complicates the fixing. With a CCD the affected frames would fallow a constant patter (a wave), so you could apply a correction bases in that "frequency" (as the Sapphire plugin does, or as an expression could do in AE).
Withe the CMOS you find that you need to fix bands inside every frame, and those bands changes from frame to frame.
I don't know if there is any filter that can fix this. There are things to fix "rolling shutter" but no sure if would help in this case.
If I can find something I'll post it.
rafael

http://www.nagavideo.com


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