Filters, such as diffusion filters or color effect filters, are also widely used to enhance mood or dramatic effects. Most photographic filters are made up of two pieces of optical glass glued together with some form of image or light manipulation material between the glass. In the case of color filters, there is often a translucent color medium pressed between two planes of optical glass. Color filters work by blocking out certain color wavelengths of light from reaching the film. With color film, this works very intuitively wherein a blue filter will cut down on the passage of red, orange, and yellow light and create a blue tint on the film.
In black-and-white photography, color filters are used somewhat counter-intuitively; for instance, a yellow filter, which cuts down on blue wavelengths of light, can be used to darken a daylight sky (by eliminating blue light from hitting the film, thus greatly underexposing the mostly blue sky) while not biasing most human flesh tone. Filters can be used in front of the lens or, in some cases, behind the lens for different effects.