Black and White Films

    Black-and-White films.  Virtually all black-and-white films for general use are panchromatic-equally sensitive (or approximately so) to the light of all colors.  The monochromatic tonal values of a subject photographed on panchromatic film will match those of the subject quite closely. 

Black-and-White film characteristics

A.    Modern black-and-white films consist of six layers, measuring a total of about .005 inches thick.  The layers in black-and-white films -- from top to bottom are: 

1. A scratch-resistant coating of hard gelatin to protect the emulsion
2. The emulsion itself, a gelatin in which the light-sensitive silver halide crystals are suspended.
3. An adhesive, a layer of glue-like gelatin to adhere the emulsion to its base.
4. The film base, a strong but flexible cellulose acetate to support all the other layers.
5. A second adhesive, between the base and backing.
6. The backing, a dye coating that prevents light passing through the film from reflecting off the camera's film pressure plate back onto the emulsion.

B.    The emulsion layer, composed of about 60% gelatin and 40% silver-halide crystals, is the most complex part of the film.  This is where an electrical / chemical reaction takes place, triggered by light, which ultimately forms the negative, converting silver halides to particles of metallic silver.

C.    The light sensitivity, or speed (ISO) of any film is directly related to the size and number of the silver halide crystals suspended in the emulsion.  Larger sized crystals are more sensitive to light than smaller ones.  This means films with larger crystals have a higher ISO rating; they are considered fast films.  The large crystal structure produces what is called "grainy" results.

D.    Slow speed films have smaller and therefore more silver halide crystals distributed throughout the emulsion.  The greater number of crystals the finer the grain.

RESOLUTION - Resolution of a film is best described as its ability to distinguish between finely-spaced lines of similar tone or density.  Slower, fine-grain films have a higher resolution.

DEFINITION - Definition indicates the quality of detail that is evident everywhere in the photograph.

COLOR SENSITIVITY - Not all films react the same way to colors.  The films used today are called Panchromatic.  They reproduce colors with brightness which is similar that seen by the human eye. And with use of different filters these films, colors can be modified to enhance or exaggerate and color.

DENSITY - Density is the degree of silver metal built up in a negative; it is directly related to the amount of light to which the film is exposed and to the length of time the film is developed.  The choice of film and developer also determines the amount of density.

CONTRAST - Contrast in film is the range between different tones of gray, specifically the difference in density between the thinnest and densest parts of the negative.  A high contrast film indicates a greater span between the gray tones, and therefore fewer shades of gray.

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