The Callier effect is the variation in contrast of images produced by different forms of illumination on photographic film. This should not be confused with the difference in sharpness (variations in partial coherence), which is also due to differences in illuminations.
The directed bright-field 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 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.