Im shooting HD green screen interview-type stuff using a couple 250w photoflex studio series lights to light the subject and the greenscreen. I know its a pretty crappy setup; ive got some 1000w w/softboxes on the way so I can light the subject correctly (the footage is very passable atm tho). Im shooting with a canon xh a1 using all manual controls.

So my problem is that when i scrub through my footage in FCP i see the greenscreen changing brightness about every 6-10 seconds then coming back to normal. Its not very noticable in real time, but it still bothers me and makes some keying troublesome. The lights don't appear to be glowing back and forth like this and the iris and shutter speed are both set manually (Ive had it happen in different settings).

Any ideas?

Sounds like you have unstable power for whatever reason. Could be something in the building is drawing lots of amps and is drawing your lights down with it.

Sounds like fluorescents with a shutter speed other than 1/60 (1/50 in PAL countries). Can you confirm that you ARE using tungstens? Were the room lights (overhead fluorescent) on?

any natural light coming in from windows or skylights? Natural light shifts. AC or Heat coming on and off can drop the line voltage as well.

It's possible ( I can't vouch for this ) that putting a surge protector between the wall socket and your lamps might solve the problem.

Are you using dimmers?

Double check to be sure you don't have auto shutter or auto white balance on. If everythng is truly manual, then it's got to be some sort of power fluctuation or fluorescent flickering. I'd use a voltmeter to check the power first.

I believe it's clear that you are using TEENAGE lights.

Teenagers historically do stuff without permission. It's why they're teenagers.

Baby lights are often quite limited. Teenage lights unruly. Only fully adult lights can be expected to give you predictable results in most situations.

When the lights you use grow up a bit, it'll be easier for you to reason with them.

Good luck.

;)

Sounds like fluorescents with a shutter speed other than 1/60 (1/50 in PAL countries). Can you confirm that you ARE using tungstens? Were the room lights (overhead fluorescent) on?

overhead flourescents were on. Im in a cramped office building where turning off the lights turns off our whole office's lights haha. Will shooting with a 1/60 shutter speed allow me to keep the overheads on? I shoot often and i can't have the whole office's lights off whenever i please; I can't move locations either.

Thanks everyone for the replies. This forum is full of video sages!

I shoot all the time under fluorescents doing 24F at a 1/48 shutter speed, and if I do interlaced, I shoot at 1/60. Any other speed might cause a flickering. However, you describe the change as happening every 6 + seconds. That doesn't sound like a fluorescent flicker to me. I'd vote for a power fluctuation.

A light meter would help to eliminate the possibility of a 60hz or camera problem. I would want to know if the actual light level is varying before hunting down electrical problems.

I believe it's clear that you are using TEENAGE lights.

Teenagers historically do stuff without permission. It's why they're teenagers.

Baby lights are often quite limited. Teenage lights unruly. Only fully adult lights can be expected to give you predictable results in most situations.

When the lights you use grow up a bit, it'll be easier for you to reason with them.

Good luck.

;)

love it!


Also, be sure your background isn't floating around, sometimes shadows can cause differences.

JS

However, you describe the change as happening every 6 + seconds. That doesn't sound like a fluorescent flicker to me. I'd vote for a power fluctuation.

If it CYCLES over a period of 6+ seconds as opposed to changing intensity, it certainly does sound like fluoros.

The non-sustained RGB nature of consumer fluoros (as opposed to sRGB pro lights) causes the light spectrum to "roll" under non source frequency exposure (ie 1/100th shutter using 60Hz power). Sorry, I should have been more specific.

As well, are you using "emergency" power outlets that are designed to maintain power in a blackout, for example: in a hospital? I plugged my lights into orange/red outlets this morning (emergency power) and noticed that intensity shifted occasionally. First time I ever noticed it.

indeed, its the flouro's causing a problem. while 24.000 is 60hz safe, 23.976 which video cameras use ( exceptions like F900/950, ect where its specific ) is not at 1/48th. if you shutter at 1/60th, all will be good again.

you could losen the bulbs in the shooting area to get them off, especially if its one room. another options is some heavy black plastic to kill them. more likely though, they are a help so just run with 1.60th and life will be good.