The Callier effect is the variation in contrast of images produced by a photographic film with different manners of illumination. It should not be confused with the variation in sharpness which also is due to differences in partial coherence.
The directed bright-field (see Fig. 1) has extremely strong directional characteristics by means of a point source and an optical system (condenser); in this case, each point of the photographic film receives light from only one direction.
On the other hand, in a diffused bright-field setup (see Fig. 2) the illumination of the film is provided through a translucent slab (diffuser), and each point of the film receives light from a wide range of directions.
The collimation of the illumination plays a fundamental role in contrast to the image impressed on a film.