Slow-Mo Sodium Vapor?

[Kip Hewitt]

I shot some 3K 36fps last night out in the street and of course all my footage has a flickering buzz to it.

The on board LCD showed none of this during the shot. Does anyone know the right shutter/fps combination to prevent this? And for future reference, is there a fool proof way to figure this out in the field?

[Evin Grant]

Those discontinuous sources are very hard to deal with. The Syncro and Phase should help but every time I try it doesn't seem to have any effect. It's
Something worth asking Red about.

[David Mullen ASC]

This chart list frame rates that are safe at any shutter angle for 60 Hz discharge lights:
http://www.cely.com/flickerfree.html

Generally if you can shoot with a 1/60th shutter speed at a crystal-sync speed, you're fine for 60 Hz lighting. So if you want to shoot at 36 fps, I guess to get 1/60th, you'd need a shutter angle of around 216 (and a fraction), which most movie cameras cannot do, but most video cameras can be set for 1/60th. I believe 1/120th also works. And 1/30th should also work, which isn't really possible at 36 fps (with a 360 degree shutter, max exposure time can only be 1/36th at 36 fps.)

There's a chart here for safe speeds in combination with certain shutter angles for 60 Hz lighting:
http://www.panavision.com.au/Information/Flickerfree60Hz.htm

[Paul Leeming]

Thanks for the links David, there's also a 50Hz Panavision page here (http://www.panavision.com.au/Information/Flickerfree50Hz.htm) for those of you using that power type.

[Kip Hewitt]

Thank you, guys. This is a big help.

[Barry Green]

Yeah, but just getting "flicker free" isn't enough when you're dealing with a rolling shutter. Rolling shutter introduces its own brand of complications when shooting under a discontinuous light source. If it's an extremely high frequency source, you're okay, but if it's a low-frequency light cycle you're going to get banding throughout the images, which will scroll up or down depending on how out-of-phase your frame rate is with the cycle of the light.

In short -- you can't shoot effectively under slow-cycle discontinuous lights with a rolling shutter CMOS camera.

[Roberto B]

when the redbook barry?..

[redrum]

That'll have to wait until I know everything there is to know about the Red One! :)

[Mike Prevette]

I experienced the issues Barry is talking about under normal florescent fixtures with a high shutter speed. YOu can see the banding in the background of the IR tests I did. http://www.reduser.net/forum/showthread.php?t=9412

[David Mullen ASC]

Yeah, but just getting "flicker free" isn't enough when you're dealing with a rolling shutter. Rolling shutter introduces its own brand of complications when shooting under a discontinuous light source. If it's an extremely high frequency source, you're okay, but if it's a low-frequency light cycle you're going to get banding throughout the images, which will scroll up or down depending on how out-of-phase your frame rate is with the cycle of the light.

In short -- you can't shoot effectively under slow-cycle discontinuous lights with a rolling shutter CMOS camera.

At 24 fps under 60 Hz discharge lighting with a 180 degree shutter, I think you get around 2 1/2 pulses per frame, 2 pulses exactly with a 144 degree shutter. I think this would be fine with the rolling shutter (otherwise you couldn't even shoot under fluorescent lights in an office or grocery store, let alone under streetlamps) -- it's when using the higher frame rates / shorter shutter speeds, you may be getting only one pulse per frame where it could become more of a problem.

[Dj Joofa]

but if it's a low-frequency light cycle you're going to get banding throughout the images, which will scroll up or down depending on how out-of-phase your frame rate is with the cycle of the light.


And,


it's when using the higher frame rates / shorter shutter speeds, you may be getting only one pulse per frame where it could become more of a problem.

Under a sinusoid illumination flicker, which is typical under 60 HZ, or 50 HZ countries, the captured image can be made "flicker-free" when the (rolling shutter) sensor integration time (exposure time) is a multiple of the reciprocal of the twice the line frequency.

Phase will not matter in the case of a continuous sinusoid as long as the above mentioned constraint is followed.

[Policar]

So I can use a magnetic ballast on an HMI and shoot okay with a rolling shutter so long as I maintain the right shutter speeds? (1/48th, 160/th, etc.?)

[David Mullen ASC]

If I understand what some people are saying here, there might be some really short shutter speeds that are safe for film but not for the rolling shutter under 60 Hz / 50 Hz AC pulsing lights. Don't know what those are without testing, maybe in the 1/120th range and shorter.

[Dj Joofa]

If I understand what some people are saying here, there might be some really short shutter speeds that are safe for film but not for the rolling shutter under 60 Hz / 50 Hz AC pulsing lights. Don't know what those are without testing, maybe in the 1/120th range and shorter.

Does not have to be short, any exposure time that is not a multiple of 1/120 (for 60Hz) and 1/100 (for 50 Hz) shall show some flicker in the case of a rolling shutter sensor.

[Policar]

So would 1/48th? (2.5 pulses?) Should I just shoot everything at 1/60th, then?

[David Mullen ASC]

Does not have to be short, any exposure time that is not a multiple of 1/120 (for 60Hz) and 1/100 (for 50 Hz) shall show some flicker in the case of a rolling shutter sensor.

I'm not talking about classic hertz flicker problems, I'm talking about the partial or "broken" frame problem that the rolling CMOS shutter causes under strobe lighting and whether it can also occur with very short shutter speeds under discontinuous AC lamps.

[JFirestone]

I have shot 75 fps 2K with fluorescent 60hz lighting, and have had to use a 120th of a second shutter to keep it from flickering. I've also shot with magnetic ballast HMI's and had success with 60 and 120 shutter speeds. The phase adjustment works well in eliminating flicker altogether. I noticed I got a very slight flicker even at 60 until I dialed in the phase adjustment.

So far I haven't noticed any rolling shutter type artifacts with either standard fluorescents or magnetic ballast HMIs.

[Dj Joofa]

I'm not talking about classic hertz flicker problems, I'm talking about the partial or "broken" frame problem that the rolling CMOS shutter causes under strobe lighting and whether it can also occur with very short shutter speeds under discontinuous AC lamps.

David, I think I understand what you are talking about, if not please correct me. Flicker can manifest itself as inter-frame artifacts, to which we are used, and for a rolling shutter camera, also as intra-frame (on a frame by frame basis).

The following is an example of the "banding effect" of flicker on a single frame of a rolling shutter camera because of 60/50 Hz frequency. I have added some contrast to make it more viewable:

http://www.reduser.net/forum/uploaded/7051_1204604209.jpg

Not exactly a tearing/broken frame problem, however, still nauseating because of change of intensity variation resulting from the mismatched timing of the rolling shutter with 60/50 HZ line frequency.

[Dj Joofa]

So would 1/48th? (2.5 pulses?) Should I just shoot everything at 1/60th, then?

I would tend to think that 1/60 is better for 60 Hz countries where you think certain lights might cause flicker. Flicker can be "hidden" in the content of an image if the content can be arranged to provide enough dithering.

In addition, the presence of dominant image signal energies near flicker frequency can also cause the flicker to be "swamped" and less noticeable.

[Policar]

Interesting; fwiw, the hvx seems to do fine with HMIs (so long as the power source is good) at 1/48th... Not that it's a huge problem; I just hate losing however much light (1/3rd of a stop or something?)

[Barry Green]

HVX is a CCD and is as such immune to rolling shutter issues. Film works fine at 1/48 as well. The question David's asking is does the rolling shutter introduce issues above and beyond what we had with film. I'll try to remember to test this out tomorrow.

[Steve Freebairn]

Under Kino lights, the Red is fine at 1/48th at 24 and 23.98, under crappy fluorescents, you'll get an image that has some bars like the image a few posts ago.

[Policar]

Under Kino lights, the Red is fine at 1/48th at 24 and 23.98, under crappy fluorescents, you'll get an image that has some bars like the image a few posts ago.

Well kinoflos have electronic ballasts so they should be fine for anything.

That 60hz sources don't work with Red is definitely problematic. I did a lot of shooting with the HVX recently and found 1/48th to work fine with everything. I don't see why a rolling shutter would introduce new flicker, particularly since film has its own sort of rolling shutter and works fine... If 2.5 pulses exposes each frame the same, why would it expose differently on a rolling shutter?

Oh well, though. Not the end of the world.

[Dj Joofa]

Under Kino lights, the Red is fine at 1/48th at 24 and 23.98, under crappy fluorescents, you'll get an image that has some bars like the image a few posts ago.

Did you try 1/60 and 1/120 at 24 fps with the florescent light?

[Dj Joofa]

If 2.5 pulses exposes each frame the same, why would it expose differently on a rolling shutter?



For a rolling shutter camera the flicker can manifest itself as "banding" even in a single image (please see the image in one of my posts above), and perhaps is what is observed by kinobarin when operating the camera at 1/48 seconds exposure time at 24 fps with florescent lights. This happens because the integration for each row happens at a different time, and is not a question of 2.5 pulses.

Unless the integration time is derived by the forumlas I listed in my posts above you may see banding from flicker even on a frame by frame basis.

In addition, there is another issue to consider -- blanking. Video cameras make it even more complex than film, because of the vertical blanking. If the vertical blanking is not properly adjusted then even that banding per frame moves up and down on each frame (phase change per frame) and becomes more nauseating.

[Mitch Deoudes]

Joofa - was your test frame shot on the Red, or another camera? Could you detail your setup for the shot (lighting, shutter speed / framerate, etc.), for inquiring minds?

Barry - interested in your test results, when you've got them.

[Dj Joofa]

Joofa - was your test frame shot on the Red, or another camera? Could you detail your setup for the shot (lighting, shutter speed / framerate, etc.), for inquiring minds?

Barry - interested in your test results, when you've got them.

We have done several tests under flickering lights; none of them with Red. I can't post our own internal images. That image that I posted was not done by us. To provide a quick illustration of how flicker can cause "banding effects" even on a single frame when the exposure time was not matched with line frequency, I pulled that image out of an academic publication, but most certainly it was not done with Red. I have to go back to the publication to see the values they used.

[radiant]

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