Callier effect

Definition: The apparent difference in density and micro-contrast seen when illumination on a silver-based negative is changed from parallel or collimated to diffused. * Also seen when dust specks in the atmosphere are invisible in diffused light but clearly visible against a dark background when illuminated by the parallel rays of the sun. * The effect is caused by different way light is scattered according to type of illumination.
Sharper light
The Callier effect explains the increase in contrast of a silver-based negative when it is printed with a condenser enlarger compared to printing with a diffused light-source or a cold-cathode light. With colour materials, the diffuse and specular densities are about equal because there is little scattering by the dye-clouds; there is no practical change in contrast with different enlarger heads. (However, you get lots of sharp dust specks with condenser light-sources.)

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