Ground Glass Q at DVinfo.net

Dave Frank · August 31st, 2004, 02:24 PM

I recently finished a shoot that involved a lot of shooting outside in bright daylight. The stuff shot outside was done half in the EARLY morning and half later in the day. I think I might have made a mistake because I was told by the editor that there was a lot of ground glass showing in the footage from early morning. This freaked me out. Luckily there was plenty of footage from later in the day that I was told looked fine. But I am now wondering what it was that I did wrong. I never stop down my lens below f/2, and always just control the light with my shutter. The only thing I can think of was maybe I adjusted the iris on the mini35, which isn't something I would normally do, but I was wicked tired.

My question is whether there is any way other than stopping the lens down that can introduce the glass. Thanks!

Andre De Clercq · August 31st, 2004, 02:56 PM

With very high shutter settings I suppose you could freeze the grounding structure and making it visible on frame by frame viewing. The structure is supposed to be blurred within the frame at normal speed settings.

Guy Genin · September 1st, 2004, 09:19 AM

Dave, it seems that indeed your controlling the light exclusively with the shutter can create a situation where the ground glass structure becomes visible. In that case the speed of the ground glass and the shutter are close to be in phase. If this is noticeable, a small change of the speed of the Mini (from 3 to 4 for example) should resolve the problem.
The question I have for you is why do you avoid using the iris on the Mini? Its location on the back of the ground glass avoids the problem of grain visibility at all apertures.

Dave Frank · September 1st, 2004, 09:35 AM

Interesting info guys. I never thought about the relationship of the shutter speed to the spinning speed.

To answer Guy's question, the reason I avoid the iris on the mini was that the first time I used the system(inexperienced, yes) I used both the iris on the mini and the shutter to control light and had ground glass appear. It scared me away from using a technique(controlling with shutter) that had worked flawlessly before. It must have been just the wrong combination of settings though.

Unless someone knows what shutter speeds are problematic with the mini35, I should do some tests.

Also, I was wondering if shooting in frame mode, along with the shutter changes, could have an effect.

Andre De Clercq · September 2nd, 2004, 06:43 AM

Although a close to synchronious sampling of the moving GG is teoretically possible, I don't think this is the case. Therefore it would be neccesary that the glass motion is close to 3600rpm (for NTSC)or a multiple of this.

August 31st, 2004, 02:24 PM	#1
Dave Frank Major Player Join Date: Feb 2004 Location: Los Angeles Posts: 244	Groud Glass Q I recently finished a shoot that involved a lot of shooting outside in bright daylight. The stuff shot outside was done half in the EARLY morning and half later in the day. I think I might have made a mistake because I was told by the editor that there was a lot of ground glass showing in the footage from early morning. This freaked me out. Luckily there was plenty of footage from later in the day that I was told looked fine. But I am now wondering what it was that I did wrong. I never stop down my lens below f/2, and always just control the light with my shutter. The only thing I can think of was maybe I adjusted the iris on the mini35, which isn't something I would normally do, but I was wicked tired. My question is whether there is any way other than stopping the lens down that can introduce the glass. Thanks! __________________ http://www.sleeptightmovie.com

September 1st, 2004, 09:19 AM	#3
Guy Genin Sponsor: ZGC Join Date: Feb 2004 Location: Mountain Lakes, NJ. Posts: 36	Dave, it seems that indeed your controlling the light exclusively with the shutter can create a situation where the ground glass structure becomes visible. In that case the speed of the ground glass and the shutter are close to be in phase. If this is noticeable, a small change of the speed of the Mini (from 3 to 4 for example) should resolve the problem. The question I have for you is why do you avoid using the iris on the Mini? Its location on the back of the ground glass avoids the problem of grain visibility at all apertures. __________________ Guy Genin ZGC Technical support

September 1st, 2004, 09:35 AM	#4
Dave Frank Major Player Join Date: Feb 2004 Location: Los Angeles Posts: 244	Interesting info guys. I never thought about the relationship of the shutter speed to the spinning speed. To answer Guy's question, the reason I avoid the iris on the mini was that the first time I used the system(inexperienced, yes) I used both the iris on the mini and the shutter to control light and had ground glass appear. It scared me away from using a technique(controlling with shutter) that had worked flawlessly before. It must have been just the wrong combination of settings though. Unless someone knows what shutter speeds are problematic with the mini35, I should do some tests. Also, I was wondering if shooting in frame mode, along with the shutter changes, could have an effect. __________________ http://www.sleeptightmovie.com

August 31st, 2004, 02:56 PM	#2
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	With very high shutter settings I suppose you could freeze the grounding structure and making it visible on frame by frame viewing. The structure is supposed to be blurred within the frame at normal speed settings.

September 2nd, 2004, 06:43 AM	#5
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	Although a close to synchronious sampling of the moving GG is teoretically possible, I don't think this is the case. Therefore it would be neccesary that the glass motion is close to 3600rpm (for NTSC)or a multiple of this.