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Enlarger
Illumination - The
Callier Effect
The
callier effect explains why condenser enlargers produce more contrast
with conventional black and white materials than diffuser enlargers.
In
black & white enlarging, contrast is relative to the amount of silver
left on the negative.
When
callimated light rays are projected through a black and white negative,
the small grains of silver embedded in the emulsion tend to scatter
them so as not as many reach the paper as one might expect. Areas
of this silver on the negative are opaque, they block and also scatter
direct light from the condenser system much more than indirect light
from a diffusion system. the final print. |
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Where
the negative is dense and the layer of silver grains is thickest
there is more scattering, while in the areas where the silver
grains are thinner there is correspondingly less. Areas on the
film which are clear and have little or no silver allow the
rays of both condenser and diffusion systems to be transmitted.
Thus with condenser systems there is a greater differential
and resulting contrast in
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Because
the dye structure in colour film is all transparent this does not occur
with colour materials. However it can occur when printing B&W negatives
onto colour paper.
Because the rays of light in the head of a diffuser enlarger are already
scattered before they reach the negative the Callier effect is not as
great. There is also no effect with a contact print as the negative
is in contact with the paper.