Re: APH & colour sensitivity


Joao Ribeiro (jribeiro@greco.com.br)
Sun, 07 Mar 1999 23:29:20 -0300


Hi Charles, Liam and all,

> 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.).
> 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.
>

I agree with the above. If we start with a magenta(100% Blue and 100% Red) than
in the shadows we have let's say 90% B and 90% R, after a 10% Cyan we would end
with a light of 90% B and 80% R.Let's say the highlight gives us 10% B and 10%
R, after the 10% Cyan we end with
10% B and 0 R. Since the red part plays no effect in ortho film the magenta
light and cyan filter did nothing different than the enlarger's light alone, in
my opinion.
But Kevin said that "The Kodak data I have show the typical ortho film, lith
2556, as being
equally sensitized to blue & green light, increasing the speed."
If Arista is different, let's say that it has different sensitivity to green
than to blue AND if the curves of the two lights are different and more friendly
in one of theese colors, than we might think of different colors giving
different rendition.
A good test would be printing with a full green light and with a full blue light
and comparing the results.
On the other hand, I have a chart of Dave's LC1 developer (fig 5 of PF#2))
giving a fairly straight line of about 16 stoufer steps! So, as I said before,
why not changing the first developer?
I remember Dave saying something that it would never reach the needed density
for reversing, but with no exposure time nor any combination of parts?

Cheers

Joao



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