July 19, 2009

How Color Film Works

Color film consists of an acetate or polyester film base with multiple emulsions coated on the base. Each emulsion layer is only sensitive to specific colors or lights. In the classic example of color sensitivities are red, green, and blue (RGB). The top layer of film is blue sensitive as all silver-based films have some sensitivity to blue light. Beneath the blue layer is green and red sensitive layers. Of course, each film may differ from the classic example and may contain multiple layers sensitive to each color, with each layer having different sensitivities to speed and contrast. Because of the complexity of emulsion layers, color film can be exposed over a wide range of lighting conditions and is much more flexible than black and white or slide films.

When the color film is developed, dye couplers within each red, green, and blue sensitive layer will produce cyan, magenta, and yellow dyes when developed, resulting in an inverse image. When printed to photographic paper, the color negative is exposed to results in the proper respective colors: cyan to red, magenta to green, and yellow to blue.

How Black and White Film Works for Film Photography

The image on a black and white film negative is actually the inverse of the actual image. That is to say, all the areas that show white on the negative will be black on the print, and all black areas of the negative will show white. When printing onto photo paper, light is able to pass through the white areas of the negative, resulting in more light hitting the paper, and leading to a dark spot. Of course, black areas of the negative are the opposite, resulting in less light hitting the paper to produce a light spot.

Simple black and white films are made of three layers. First, there is a light-sensitive emulsion layer that captures the image that will result in the film negative after processing. The emulsion contains grains of silver salt that are able to absorb light and react with a developing chemical to break down into pure silver and remove it to reveal an image on the negative. Second is a layer of plastic to support the emulsion. Third is an anti-halation layer that is used to capture light and refrain it from bouncing back to the emulsion. This final layer eliminates blurry images or foggy film.

The amount of silver salt and grain in black and white film decides whether the film is more or less sensitive to light. Fine grain will require more light to produce an image and large grains will require less light to produce an image. The amount of grains on the film help determine the ISO film speed of the black and white film. Fine grain leads to slow film and large grain leads to fast film.

How Color Reversal Film Works

As the name reversal suggests, slide film works the opposite of print film. In print film the red, green, and blue emulsion layers are exposed and leave a negative dye of cyan, magenta, and yellow. Slide film is a subtractive process that starts with layers of cyan, magenta, and yellow. When the film is exposed, the dye is subtracted to reveal red, green, and blue colors.Thus, when processed the film reveals the actual, positive, colors of the image.

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