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D.P. Journal

ND Issues Abound

Jumping a bit forward into one of the major productions snags we hit. The HVX has a sweet spot to maintain maximum resolution. The resolution begins to degrade above 5.6 or so (maybe a little higher). I do not fully understand the mechanisms why but I beleive it has to do with wavelengths of light and the small aperture of a 1/3 inch chip camera. Nonetheless, I made a rule that we would not shoot above 5.6 so that we could maintain maximum resolution for a film out process later on.

To maintain proper light levels I had a full arsenal of Tiffen ND filters on hand to slide into the matte box. If the light levels rose above 5.6 I could bring them down with ND filters. Seems correct - doesn’t it? Until we hit our snag.

Anytime we used ND .3 we were fine. The ND did its job and we noticed nothing strange. However, at a particular location, we needed to use the ND .6 filter. It just so happens this location was the same location that our video tap cable was beginning to go and we were monitoring on the green channel only for much of the time (the HVX component connector is terrible). Not having a spare cable on hand we moved forward.

The cable occasionally worked in full color and at one point in the afternoon the script supervisor noticed a color shift. The white walls has a pinkish tint. The green towels were dark magenta (yes, magenta). However, skin tones seem unchanged. The pink shift on the walls was more pronounced in the highlights. Our shoulders slumped as we wondered at what point in the day we slid the filter in place or when the issue started. Luckily, in retrospect, the sequences were all consistent within themselves so we did not lose any footage to this issue.

But the question still stood. What was causing this bizarre and seemingly selective color shift. We spent about a half hour checking over the camera. We could reproduce the issue by sliding the ND filter out and then back in place - the issue was clearly linked to the filter. However, the in camera .9 ND filter did not cause the same shift. So either it was the Tiffen filters or it was the interaction of the Tiffen filters with the HVX.

Move forward a week and we were shooting on a bright sunny day in a bathroom with large windows. We had put ND gels on windows before but this was the first time the intensity of the sun required 1.8 of ND to get it in balance with the interior lighting. After 2 hours of putting up gels we looked at the monitor. Holy smokes - We could not believe it. The light coming throught the windows was now tinted pink. Pull the gels off and it was white. So again, either it was the roscoe gels or it was the interaction of the gels with the HVX. We had to move on and therefore we had to block off the windows and frame them out of the shots. By manipulating the action we managed to get great footage even with this limitation. But the question still remained - Why was this happening?

In posting this issue to many message boards lots of theories came up. People asked about white balance. When the problem occurred we white balanced both before and after the gels/filter was used to no effect. We could white balance to the pink area but the rest of the frame would have been green. Perhaps a camera with a paintbox could have overcome that. We were also asked about the quality of the gels and filters. Roscoe and Tiffen are not top of the line but it is reasonable to expect a good color balance in their materials.

The best explanation I could get was that there is no true ND material. It is only ND relative to the chip or film emulsion that is picking up the light through it. So the internal HVX filters have the proper colormetry and do not cause a color cast. But other materials colormetry will differ and therefore can introduce a color cast.

Here is a thread on DVXuser I started about this issue. It does get a bit heated between another user and myself - but there was a lot of great ideas and input from other people.

Morals of the story. Do not assume ND is truly ND at high amounts for your camera. Always have a spare monitor cable on hand in case the primary dies. Finally, even test the small obvious things such as color temperature when using ND filters.