Black band rolling up screen. Issue with certain lights?

[The Irish TRex]

Sometimes when we are shooting an event in a dimly lit room theres a faded black band that rolls slowly up the screen and then repeats over and over non-stop. The higher the shutter, the thicker and more visible the black bar is. If I drop the shutter down to maybe 1/40 it's almost imperceptible. But we shoot at 60fps so the lowest I'd like to go is 1/60th and even then the bar is pretty hard to see but it's definitely there. This seems to be the case with not just my Sony A7s camera, but other DSLRs as well. I'm assuming this has to do with the shutter not mixing right with the specific lights. It's not all events. Maybe one out of 5 or one out of 10. But when it happens it's a pain. Last time I was shooting an event when the room was lit and had this issue, I checked and the photographer's Canon DSLR had the same issue when he put it in video mode as well. I'm guesisng it's an issue with all CMOS sensors since my previous CCD sensor did not have this problem ever.

1) What exactly is causing this?

2) Any way/tricks to get around this while still maintaining 60fps and a minimum shutter of 1/60?

Thanks!

 

[edinc90]

The thing causing this is the ballast in the lights in the room. Fluorescent, certain LEDs, and high/low pressure sodium lights all have ballasts to provide the correct amount of power to the lights. Electricity has a frequency (60Hz in the US) which is converted to something higher by the ballasts. Add to that the fact that tungsten in traditional lights takes a long time to change brightness (long as in .2 seconds) and the gasses inside sodium and fluorescents respond almost instantly to changes is why you can see those black bars.

The only thing to do is change your shutter speed until you can't see the bars any more. Or change out all the lights for studio lights.

 

[The Irish TRex]

Would turning off E-Front Curtain Shutter or anything like that have any effect? I didn't try it last time this happened. I'm so used to my CCD camera not having this issue that it's taken me totally by surprise as 2 of the last 5 weddings I've shot these past two weeks have had a reception lit by lights that did cause this issue. I wish I could change out the lights, but these are weddings and well, it's not possible.

 

[AndrewGentle]

You're right about CMOS, this is another issue caused by rolling shutter. The lights flicker at a higher frequency than your shutter speed, as edinc90 said, and the sensor sees the brightness of the lights at different points in time as it scans from top to bottom. I've found this a particular issue with stage LED lighting, which universally use pulse width modulation to give the illusion of reducing brightness. Different lights seem to suffer from this to different degrees. I have two sets of LED stage lights, one of them causes no flicker at all, the other requires me to slow down the shutter speed until they appear constant. Shooting a stage play last year, I found that I had to increase my shutter from 1/50 s (180° in PAL) to 1/42 s to make the LEDs look good. Obviously there are motion-blur-related reasons why you'll want to slow the shutter speed as little as possible (although it is good for theatre to improve your sensitivity a little!) Unfortunately, no, there's no other way to fix this issue for Sony NEX and the higher frame rate you want, the more problems you'll have.

 

[Ebrahim Saadawi]

What you're seeing is flickr. It happens to any sensor when the shutter speed doesn't match the light frequency. Just play with the shutter speed in small increments until you find the specific one where it disappears, and with the frame rate (50p).

 

[paulears]

Probably NOT the cause here, but worth mentioning that a very similar thing can occur when using camera plugged into mains power. Ground loops cause hum in audio systems, and are a constant issue for audio people, but the same thing can cause hum bars to travel up and down the screen. Usually when people are connected to projectors, or have an audio feed from a mixer plugged into the camera. Sometimes not noticed until the screen is bright and white, when you see these feint bars going up and down. Unplugging and running on batteries usually sorts this one out.

Lighting wise, any of the discharge sources used commonly nowadays do it and LED kit turned down to very dim are very flickers - some even to the naked eye!

 

[timmytimetravel]

I had this the other day. I'm new to using the A7s (have an a7 and an FS700), and luckily I tested the A7s before using it on a shoot and saw this issue. Dropping the shutter down (in my case below 50) pal land Sorted it. The lights were not anything special just some probably tungsten practical (high ceiling)s. My very first thought was... Lucky the sun creates no such issue. Getting the A7s down to 50 outside is a bit of a task. Took me 3xND4's to be able to shoot reasonable apetures.. So sensitve!

I'm wondering why I've not seen it really in the FS700 before. I've seen banding from standard fluros in HFR (and then turn them all off and substitute kinos). I'm thinking it might also be exagerated by the high ISO? (think I was at like 100k)

If you have that to worry about for weddings make sure you check out the post where you need to rip all the blue led's out of the weddings dj's cold dead hands, option for lights or disclaimer and exception for your dance floor scene!

 

[Bruce Watson]

Sometimes when we are shooting an event in a dimly lit room theres a faded black band that rolls slowly up the screen and then repeats over and over non-stop.
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1) What exactly is causing this?

User error. You are describing what happens when you don't match frame rate and shutter speed to the flicker rate of the lights. Dumb old building fluorescents flicker as a function of mains frequency. In NTSC land, that's typically 60Hz. In PAL land, that's typically 50Hz.

There are a number of ways around this, but assuming you don't want to bring your own lights and overpower (or just turn off) the building lights, the easiest thing to do is to adapt your setting to appease the existing lighting. That means frame rates at an even multiple of mains frequency, and also shutter speed. For example, in NTSC countries, that would mean 30 or 60 fps (you get the idea) and 1/30 or 1/60 shutter speed. What this does is expose each individual frame under the same part of the lighting cycle as every other frame. If all the frames see the same amount and quality of light, no banding.

That said, there are plenty of threads on DVXuser.com that discuss this phenomenon. Maybe you can find a solution that fits your particular situation by searching through them. IDK.

 

[The Irish TRex]

Interesting stuff. Frustrating too. I'm in Southern California so I'm guessing the frequency is typically 60Hz here. I'm shooting at 60fps with a 1/60 shutter usually for these receptions and the issue is there. We have our own lights which I'll use but I can't exactly turn out all the lights at a wedding just for the video. I did crank them up a bit more last wedding and it overpowered the ambient lighting and helped fix the issue. Temporarily. For the duration of the toasts anyway. I might have to make some exceptions and shoot 24p during the reception with a 1/40 shutter. It's better than 1/30th at 60fps. Normally I like the option of slow-mo in post, but for these weddings I'll have to change up my style a bit. Probably best to include this in the contract. I wish I knew exactly what lights were causing it. I think they are LEDs or the dimming of the chandeliers above...at least if I knew I could inform couples ahead of time that "If you have _____ lights, this issue may occur."

 

[dapoopta]

Or just say "various lighting can cause a slight flicker and may modify the look of the final product. This is unavoidable and out of the videographers control". Just like some people go crazy about rolling shutter and flashes at weddings... Some compromises need to be made

 

[The Irish TRex]

Or just say "various lighting can cause a slight flicker and may modify the look of the final product. This is unavoidable and out of the videographers control". Just like some people go crazy about rolling shutter and flashes at weddings... Some compromises need to be made

I just wish I could say which specific lighting or lights would cause that so they could look into it ahead of time and see if there's other options available since usually it's still a year or so away and there's time to make changes.

 

[Bruce Watson]

Interesting stuff. Frustrating too. I'm in Southern California so I'm guessing the frequency is typically 60Hz here. I'm shooting at 60fps with a 1/60 shutter usually for these receptions and the issue is there.

Oy. You're doing the right thing by any fluorescent lights. If you still have this problem, it's coming from some light that's variable -- almost certainly cheap LEDs. They "dim" by changing their duty cycle, which means that their flicker frequency is independent of mains frequency. If this is the case, you'll want them either full on (un-dimmed) or full off. Anything else is a crap shoot, and will almost certainly cause this kind of banding artifact. That said, there are LEDs that dim just fine and don't cause video problems. I don't know a good way to tell which is which other than testing.

We have our own lights which I'll use but I can't exactly turn out all the lights at a wedding just for the video. I did crank them up a bit more last wedding and it overpowered the ambient lighting and helped fix the issue. Temporarily. For the duration of the toasts anyway. I might have to make some exceptions and shoot 24p during the reception with a 1/40 shutter.

If your problem is building fluorescent lights, going to 24p and 1/40th will almost certainly give you this banding artifact.

Probably best to include this in the contract. I wish I knew exactly what lights were causing it. I think they are LEDs or the dimming of the chandeliers above...at least if I knew I could inform couples ahead of time that "If you have _____ lights, this issue may occur."

Good idea.