Problems Using A 45 Degree Shutter With Monitors

Hi All,

I'm thinking that this is a total pipe dream. I have a studio job coming up this week using the SR3 - I want to use the 45 degree shutter for the crispy look. The only drag is that there is a small (8") practical TV monitor that is part of the set. For the 2 shots that it appears on camera, it is functional with an image on the screen.

Is there any way to shoot with the 45 degree shutter and also shoot this standard NTSC monitor? A slight roll bar is acceptable to the director. Because the time needed to shoot a test is not available we may just comp-in the image on the TV and motion track it for the brief appearances it will make.

Also, there is going to be a strip of 4' x2 household fluoros mounted in the set (w/Optima 32's). Should I worry about flicker since these aren't electronic ballasts?

Any thoughts would help,

Thanks

Jeremy Benning

Couldn't you just shoot those two shots of the monitor at 144 degrees, which will give you the smallest roll bar at 24 fps?

The problem with shooting HMI's and fluorescents at 45 degrees is not flicker, but exposure. As long as you are running at 24 fps crystal, you will not get a variation per frame that will appear as flicker even at 45 degrees. HOWEVER, while at 180 degrees, you generally record about 2 and 1/2 light pulses per frame, and 2 pulses at 144 degrees, and 1 pulse at 72 degrees (I think), at 45 degrees, you are recording less than a full pulse per frame. What this means in practical terms is that each time you roll the camera, you might catch that pulse at any point of its wave -- so sometimes you risk getting in sync with the bottom of the wave, causing underexposure.

I might have this wrong, but I suggest that you don't rely on these fluorescents to provide any sort of key exposure if you run the camera with a 45 degree shutter.

David Mullen

Another problem with monitors and moving cameras is the shearing of the TV image as the scanlines go down the screen. It could be that a tighter shutter angle would help this, but its hard to judge without testing. If in doubt, avoid fast pans.

If there is the possibility of doing the TV monitor inserts in post, it would make life much easier for you and them to either turn the monitor off, or ideally replace it with backlit blue/green in a monitor shell, particularly if anything is likely to come in front of the screen. Don't use a blue feed to the monitor - this is worse than having the monitor off, because you then have roll, shear and spill to deal with in post, as well as an unsteady image for motion-tracking purposes.

Does anyone have experience of using synchronised 24fps displays? I wonder why this technique isn't more widely used for shooting screens - it must be cheaper than replacing things in post, although you don't get the luxury of editorial control.

Tom Debenham
Computer Film Company, London

While we're on the topic of shooting monitors I'm wondering what determines whether you see the scan line in the viewfinder at the same time as the film image. A few years ago I shot several monitors with a Panavision 16 system and thought the line was moved off frame only to find it in plain view when we went to transfer. I was told later had we shot with an SR we would have "seen" the line if the film were seeing it. If that's true then what is the difference in the viewing systems?

What about Aaton?

Thanks,
Randy

It's actually quite simple. What you need to remember is that your eye and the film don't see the same thing time wise. Your eye sees the pulldown interval between film frames. Assuming the camera is synched (sunk?) with the monitor, it all boils down to whether the monitor is displaying an interlaced picture or not.

If the display is interlaced, then what you see is what you get, effectively. Your eye sees 1 field, say the even numbered lines, and the film sees the other; the odd ones. So, lose the bar in the finder and you've lost it on film.

If the display is non-interlaced, then the worst image in the finder will give you the best image on film. This must be what happened in your case. By losing the bar in the finder you had, in fact, put it into your film frame.

I'd love to be able to tell you that the only camera to use for this is the SR, but I can't.

Any camera with a reflex viewing system should behave this way.

Hope this helps,

John Duclos (Technical Manager)
ARRI MEDIA

Yes and No. depends on what frame rate you are shooting also. As with Computer monitors one tends to shoot at 1/2 the refresh rate (which is usually around 75 hz) for about 37.5 f.P.S.. I've done this, moved the roll bar off frame in the viewfinder, and had no roll bar on the film. I believe Computer monitors are Progressive and not interlaced. I've done it this way, no problems.

If I were to check my source info ( the Arri 35 book by Jon Fauer) it would tell me that shooting an NTSC monitor, at approximately 29.97 (playback from consumer vcr's tend to be slightly slower) with a 180 degree shutter will give me no roll bar when I position the roll bar off the screen in the eyepiece. However shooting at 24 FPS ( 23.976) with a 144 degree shutter will give me no roll bar when I see them on the T.V. Screen. Pages 190 and 191.

Tote that book around a lot, excellent resource.

P.S. As John alluded to, the rules in the book still apply even when using an Aaton.

Steven Gladstone
Brooklyn, Based Cinematographer

This part is incorrect based on my own personal experience. The 144 degree shutter angle merely reduces the size of the roll bar to a thin line -- itstill rolls without a film/video sync box. And at that speed, you actually have THREE roll bars -- either you can set it with the sync box so that youhave only one visible in the middle, or instead two visible, one at the top and one at the bottom third of the screen.

The only way to have NO roll bar is to shoot at the NTSC speed (29.whatever) or use 24 frame video monitors.

Maybe I'm wrong and that the key is getting precisely 144 degrees in order to have one roll bar instead of three, but the sync box is still necessaryto stop it from drifting and to phase the bar out.

However, in wide shots, just using a 144 degree shutter angle gives acceptable results since the roll bar is so small.

David Mullen
Cinematographer / L.A.