What is a Photographic Film?
A photographic film is simply a sheet of plastic coated with a silver halide emulsion. The silver halide emulsion contains silver halide salts that are bonded by gelatin. A regular photographic film used for black and white photography is made up of the components mentioned ahead.
► The Base: The base of the film is made up of cellulose acetate and can be transparent, translucent, or even opaque. The primary function of the base is to support the emulsion in place.
► The Emulsion: The emulsion layer is a thin layer of gelatin containing the light-sensitive silver halide salt crystals. The size of the halide crystals determine the sensitivity, contrast, and resolution of the film.
► The Anti-halation Backing: The anti-halation backing layer is primarily a part of the photographic film to prevent the reflection of light from the base into the emulsion.
► The Overcoating: The overcoating layer is a clear, protective sheath made of gelatin that protects the film from physical damage, like scratches or abrasions before development.
► The Non-curl Coating: The hardened gelatin layer applied at the back of the film is called the non-curl coating. Since the emulsion swells when it is wet and shrinks when it becomes dry, there is a resulting strain on the flexible film base, which can result in curling. The non-curl coating prevents this curling.
Role of Silver Halide in Capturing Photographs
Silver halide is light sensitive and hence exposure to light causes a chemical change in the silver halide salt crystals. This reaction that occurs due to light exposure helps in the formation of a latent image on the film.
This latent image is called the negative image, which can then be processed further with additional chemicals to obtain a positive copy or the developed copy of the photograph.
Film Developing Process
There are several development processes that can be used to develop the photographic film. Most of these methods only differ in the containers used to carry the chemicals but more or less use the same chemicals in the actual process.
There are several processes like the tank method or the tray method for film development. Here is the description of the tank method for the development of the photographic film:
This is the pre-soaking stage. Put some water in the tank, and soak the film for a few minutes. The time for which you need to soak the film will differ with the size of the film. After the soaking, pour the water out.
The developer has to be diluted with water according to the standard measurements and then poured into the tank. The diluted developer should now be poured into the tank carefully to completely submerge the film. Once the developer is poured, start the timer and close the push-cap on the tank.
Shake the tank by flipping it over for ten seconds after every minute. As the timer rings, pour out the used developer down the drain. Always remember that soaking the film in the developing solution is a crucial stage, and the timing for which the film has to be soaked in the solution has to be accurately followed.
Use the stop bath solution (acetic acid) as a stop bath. Fill the tank with the stop bath solution, shake it, and pour out the contents. Repeat the procedure twice.
The fixer helps to fix the image on the photographic film. Pour the fixing solution in the tank and shake it for ten seconds after every minute. Continue the fixing process for 5 to 10 minutes and rinse the rank after that.
Once the fixing process is over, the film can be safely exposed to light. Unscrew the top of the tank and take out the reel. Now, fill the tank with water and shake it for half a minute and pour the water out. Repeat this procedure for the following 10 minutes. Make sure there are no remnant traces of the fixing agent since that can ruin the entire process.
Pour a small amount of wetting agent into the bottom of the tank, and fill the tank with water until the reel is submerged. Ensure to shake the bubbles off the surface of the film and leave the solution for about 30 seconds. After that, pour the contents out and take out the reel. Do not by any chance rinse the film now.
Lift the film off the reel and allow it to unfurl. Make sure that it doesn't touch the ground or anything else. Hang it up for drying for 4 to 8 hours so that it has sufficient time to dry and harden.
Once the negative is ready, it has to be processed and transferred onto a photographic paper. The enlarger, which is an optical apparatus, is used to project the image of the negative onto a base and finely control the focus, amount and duration of light incident on the paper.
A sheet of photographic paper is exposed to the enlarged image that projects from the negative. During exposure, the dodging and burning techniques can be used to adjust the values of the image. These processes include reducing and increasing the amount of incident light selectively for a part or the entire exposure time.
After exposure, the photographic printing paper is ready to be processed. The photographic paper is processed using chemicals in the following order:
- The print is developed using a photographic developer
- Rinsing with stop-bath
- Fixing the image permanently with the use of photographic fixer
- Washing to remove all the processing chemicals and then finally drying it