RE: Colorsync-users Digest, Vol 2, Issue 123
RE: Colorsync-users Digest, Vol 2, Issue 123
- Subject: RE: Colorsync-users Digest, Vol 2, Issue 123
- From: "Mark Rice" <email@hidden>
- Date: Sat, 26 Mar 2005 19:25:37 -0500
- Organization: Zero One
No, chromes are exposed to the silver contained in them. The silver is
"coupled" to a dye. The dye is formed during color development, and then all
the silver is bleached out of the chrome.
The silver effect that drum scanners suffer from is called the Callier
effect. This is also the process that makes a condenser enlarge more
contrasty than a diffuser enlarger. Drum scanners use point light sources,
but flatbed scanners and densitometers use diffuse light sources.
More on the Callier effect:
Callier effect is just a fancy name for the well known fact
that optical density of silver based black and white films
is a function of the "directness" of the light. They let more
of diffuse light through than collimated light.
The important thing to remember is that densitometers use
diffused light. Hence, if a film is said to be able to reach
an Dmax of 2.3 it really says that the Dmax is 2.3
with diffused light. Different films react in different way
when the light is made more collimated. The difference between
the optical density is sometimes given as a number called
Callier coefficient or Q-factor.
Example:
Assume that a film has a Q-factor of 1.7 for given light source.
Then an area of the negative that measures 1.0D actually
lets through that kind light as if its density was 1.7D.
Dye based materials have a Q-factor of 1, i.e., they don't
care what type of light is used to enlarge/scan them.
Basicly the light sources used in enlargers fall to three
basic categories of "directness", from most diffuse to the
least diffuse.
1. Diffusor heads (color heads and built in VC filter heads)
2. Condenser heads (many 35mm enlargers)
3. Point light sources
Point light sources are not used in ordinary darkroom
b&w printing, but only in some (non pictorial) special
cases requiring utmost sharpness.
Now to scanners: Lets imagine we have scanner that is capable of
DMax of 3.6. However, it uses a point light source. Now, we want
to scan an ISO 400/27 silver b&w film with that scanner. All of the
information we want to scan has an optical density between
0D and 2.3D. So far it looks good, but now it turns out that
this is a pretty grainy emulsion, and for this kind light source
its Q-factor is 1.7. Since 1.7*2.3D=3.9D this means we are out of
luck, we can't get both highlight and shadow information with one
scan.
> It is considered that the point light source of the analysing lamp can be
> diffracted by the silver grains in the wedge in such a variously
> unpredictable manner, as you step from sample to sample up the density
> scale, that it becomes difficult or next to impossible to accurately
measure
> the density/dot% of the original wedge and predict the results of the
> eventual output.
>
> Richard
Yes, but aren't most chromes silver-based?
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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