[tig] b+w stock question

timba at eurobell.co.uk timba
Mon May 3 11:14:42 BST 2004


One thing to consider when using colour stocks to produce B+W from most
colour telecines is the failure of "the principle of constant luminance",
which is a well known deficiency/distortion to students of such matters.
What I mean by that is that due to the way gamma correction is applied in
the video chain of a telecine or camera the brightness (luminance) level of
coloured areas will be lower than they should be when looking at the B+W
output of the telecine or on a B+W monitor/TV. This is because the matrix
which produces Y (luminance) from RGB in a telecine or camera comes after
the gamma correction. If there's a matrix to produce Y, just for B+W,
before gamma correction then this distortion doesn't exist.

I guess most people would produce B+W from a colour film on a colour
telecine by switching the chroma off, turning the saturation to zero or
just using the Y signal from the telecine output. However some telecines
have a Colour/Mono switch which switches in a pre-gamma Y matrix. The old
Cintel MkIII telecines have one although it will only work properly when
transferring print, not neg. I have a feeling that the URSA doesn't have
one and don't know about other models. 

So if you feel that the look of your monochromised pictures are not quite
right it could be because of this. The effect is not insignificant: for
example if you take the case of full amplitude 100% saturated red the
brightness displayed will be aproximately a quarter of the correct value.

Timothy Balenger
London SW

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