Photographic film is one invention that has changed the way people see the world. Because we are able to take photographs, we are able to capture memories and see places in the world that we would not normally have the opportunity to see. Photographic film reacts only to light exposure. Differing levels of light exposing black and white film determines black areas, white areas and gray tones to create a negative image on the film. The positive image is not created until the negative image is transferred to the photographic paper, but that is a different story.
The Base
The base of the film, which forms the structure of the film, is a transparent celluloid material that is 4/1000 to 7/1000 of an inch thick. One side of the film is shiny and the other side is opaque. This difference serves handling purpose during both the manufacturing process and the developing process.
Photochemical Side
The photochemical side is where all the magic happens. Up to 20 different layers are added to the photochemical side of the film to create a total thickness of about 1/1000 of an inch. Gelatin, the same ingredient that makes gelatin desserts, acts as the binder to keep each layer in place. Gelatin has been used in this capacity for more than 100 years.
Silver-Halide Crystals
Not all of the 20 photochemical layers help form images. They control chemical reactions and filter light. The layer that actually creates the image is embedded with silver-halide crystals that are light sensitive. Because these crystals are light sensitive, they are the workhorses of photographic film and act to create an image by reacting to different levels of light to create dark and light areas of the image. These crystals are made by combining silver nitrate and halide salts in an extremely complex chemical process. The crystals differ in size, shape and composition. Then they undergo another chemical process to make them even more sensitive to light.
Spectral Sensitizers
Before silver halide crystals go through the second chemical process to make them more sensitive, they react only to blue light. The second chemical process adds spectral sensitizers, which are organic molecules, to the surface of the crystals making them sensitive to red, green and blue light. Other chemicals are added to the silver halide crystal-making process to determine the sensitivity of the crystals. This sensitivity determines the exposure speed of the film, such as ISO 100, 200, 400 or 800.
Exposure Speeds of Film
The exposure speed of film determines the usable conditions for that film. Use 100 speed film outside in bright sunlight. Use film with 200 speed outdoors or inside with bright lights. Use 400 speed film for indoor photography; bright light is not necessary if a flash is used. A film with 1000 or 1600 speed is for indoor photography when you do not want to use a flash.
Lower film speeds also require subjects to hold still while the film is exposed, to avoid blurring. High-speed films are also good for catching action, like at sporting events.
Tags: chemical process, speed film, 1000 inch, blue light, chemical process make