Avoiding refresh rate flicker when filming screens - Canon 700d

Hi all,

I am going to film a video installation and want to avoid screen flicker.

The installation is a 180-degree dome with video projected inside and controlled by the viewer with a controller to create a 'virtual reality' walkthrough environment.

I use a Canon 700d (Rebel T5i) for filming HD video.

I know that others filming the dome have had trouble with flicker appearing on their video before so I wonder if anyone can advise on how to avoid this problem?

Many thanks,

D

Refresh rate frustration usually only comes into play when shooting a CRT monitor. That's when you get the exciting rolling bars. I've never been able to get rid of them completely on a DSLR. On a projected screen there will be none of this. There's no shutter like on a film projector so you won't be capturing moments of darkness resulting in flicker. Depending on the frame rate of the projected image you may get the occasional "double frame" which will be blended together but it's really not that objectionable. If you are shooting 30fps for example and the projected image is the same, it should look fairly good.

You want the shutter speed and frame rate of the camera synced up to the refresh rate of the projector. So usually 60fps at 1/60th with the projector at 60hz, _should_ work well.

In post, you can also use a plugin we make called Flicker Free if you do have rolling bands/bars. It's usually quite good at eliminating the problem. There's a free trial available at:

digitalanarchy.com/Flicker/main.html

However, getting the camera in sync with the projector will keep the problem from happening in the first place. Use a multiple of the refresh rate.

cheers,
Jim

DMitchell wrote: ...I am going to film a video installation and want to avoid screen flicker....The installation is a 180-degree dome with video projected inside and controlled by the viewer with a controller to create a 'virtual reality' walkthrough environment.

I use a Canon 700d (Rebel T5i) for filming HD video...

As Redifer said "refresh rate" generally applies only to phosphor-based CRT displays. In that case a hardware genlock signal between the devices prevented this: en.wikipedia.org/wiki/Genlock

With digital projectors or panels there isn't refresh in that sense, so this is less a problem. However in live video, genlock is still used to avoid a glitch when switching video sources.

Whether shooting DSLR video of a digital screen or anything else, it's a good idea to either use either (a) manual exposure or (b) shutter priority, with the shutter speed at the reciprocal of the frame rate. E.g, for 30 frames/sec use 1/60th sec. This is because of the "180 degree shutter rule": cameradojo.com/2014/03/28/why-shutter-sp...ers-with-dslr-video/

If manual or shutter priority is not used, the shutter speed can fluctuate. This can cause flicker, strobing or bands. Banding can also happen from fluorescent lights, which are being refreshed at the AC frequency rate (50 or 60 Hz). Usually setting to 1/50th or 1/60th avoids this.

This can be an insidious problem. A brief test shot and playback may not show anything if the auto-selected shutter speed is in a certain zone. Then when the actual shot is done, if lighting changes the shutter speed can change into a zone causing interference banding. You can shoot a lot of material and only see then when too late.

If the projector lamp uses pulse-width-modulation for brightness control, it's conceivable this could cause a problem depending on the pulse rate. You'll have to shoot test footage to evaluate this.

If there are any safety or pathway lights in the theater, these might also use PWM dimming, so could be a problem.

These effects are often difficult to see when played back on a camera's small LCD screen. Ideally you need a larger field monitor or a laptop to download some test material and evaluate.

As anarchyjim said, Digital Anarchy "Flicker Free" tool is amazing and can fix otherwise unusable footage. It is very CPU intensive to run but it does a great job and is easy to use. However it's best to avoid the problem in the field, if at all possible, vs fixing it in post.