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Forced Pedestal in Camera


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#1 Anna Uio

Anna Uio
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Posted 06 December 2007 - 11:36 PM

Hi all,

After a number of experiments, I've come to the realization that all of my video cameras shift the black point to zero and that there is no way to disable this "feature". This effect is often referred to as left-justifying the histogram. While it may be a useful creative choice, it is certainly not something you want forced on you, as it will make color correction in post impossible. Each frame gets offset by a different amount depending on the level in darkest part of the frame, and any offset other than zero will make it not possible to use RGB gain perform color balance. Nor will any other color correction method likely work across a whole scene from beginning to end with varying light levels.

The image below shows an example of this behavior. The curtain was lit from the left and the right by identical light sources, one farther from the curtain than the other. So both the light and shadow parts of the curtain are receiving considerable light, and there is nothing black in the scene. Looking at the RGB histograms, you can see that they have obviously been left justified so that the minimum (blue channel) becomes zero. This very much changes the image, causing the shadows to become much, much darker and more saturated than they should be in this light filled shot. Note that film would not do this. It is a very artificial thing to do.

I don't know what to think of this. It seems awful. The cameras I've tested are Sony Fx-1, Sony Hc-1 and Canon Gl2, and they all do it.

I'd be curious to hear comments/thoughts from other people who regularly works with color adjustment in post.

Cheers,
A

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