Charles Steinmetz (csteinmetz@redneck.efga.org)
Sun, 07 Mar 1999 20:15:23 +0000
Liam wrote:
>Now interpose a FAIRLY PALE cyan filter between lens and baseboard. My
>point is that a pale cyan filter would have negligible effect on the colour
>of (strong) light passing through the shadows of the negative - which is
>thus still magenta when it reaches the baseboard - but a much larger effect
>on the colour of the weaker light that has passed through the highlights of
>the negative, which would become more or less blue. This colour change is
>brought about by the cyan filter subtracting or holding back a good
>proportion of the red component of the (weak) magenta light but passing the
>blue fairly freely. And in this way I think it may be possible to print the
>shadows by magenta light (of relatively low actinic power), and the
>highlights by blue, the effect being to lower contrast.
Liam, I think this assumption is incorrect, which is what my thought
experiment was intended to show. Filters do not treat bright light and dim
light differently. If the negative is neutral, the color of the light at
the baseboard will be the same in all areas (determined entirely by the
light source and the two filters, not at all by the negative). There is no
magic in where you put the density in relation to the filters. You can
verify this with a color densitometer, if you have one available: Build a
sandwich of a strongish magenta gel filter, a step wedge, and a weak cyan
gel filter. I am quite sure you will find that the tricolor densities of
each step are the same, regardless of where the step wedge is in the
sandwich. Only if the negative itself changes color with density will the
light at the easel have a different spectral composition from one density
area to another (imagine a CC05C over one step, a CC10C over the next, a
CC20C over the third, etc.).
>A practical application of a vaguely similar principle occurs to me. Years
>ago, Cokin used to market pairs of complementary-coloured filters ***, one
>for use over the camera lens and the other over the flash. The idea is that
>with, say, a blue filter over the lens and a yellow over the flash, the
>background (where the flash is ineffective) is rendered blue, but the
>colour is cancelled out by the yellow flash on the main subject and
>therefore comes out neutral (on colour film, of course!)
This is true because the background areas are lit by some non-flash light,
which is not corrected and therefore appears to be the color of the filter
on the camera lens. (If you used the Cokin system in a totally dark room,
the background would only be dark, not blue [unless the film itself tended
towards blue shadows, which many reversal films do].) The twin-filter
contrast-control scheme has no such other light.
Best regards,
Charles
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