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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

 

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.

 

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