It is not a necessity to know every chemical and physical component of film to be a good photographer, but I think most great photographers and very intimate with their medium. By knowing the underlying details about film properties would indicate you have more knowledge and ultimately more opportunity to tune and exploit the variety of details in your photographs. My goal with this brief article is to quickly summarize the various components that make up black and white film.
Film – What’s it made of?
If you could look at a highly magnified version of a piece of film from the side you would see several different layers that go into making the film we use in our cameras. Refer to the image at the top of this article when reviewing the proceeding paragraphs.
Using the reference numbers noted in the illustration above I will provide an overview of each layer. In general when you think of film you should understand there is a base and an emulsion. The base is the transparent material that supports the coated emulsion layer and you may have guessed the magic happens on the emulsion.
1.) Anti-Scratch Layer. This clear layer of gelatin does exactly what its name implies. It helps protect your film from scratches that could occur before you develop your film. And, yes the gelatin comes from the same place as Jello!
2.) Emulsion Layer – The emulsion layer of gelatin is literally suspended and supports the magic photography dust called silver halides. Silver halides often referred to as crystals is really a group of compounds that include silver with bromine, iodine, and chlorine. When the crystals are exposed to light they are reduced to black particles of metallic silver when you develop your film. When you take your exposure a latent image is literally formed on the crystals that is invisible until the developer makes it react. The more exposure you have in one area the more silver you will have when developed causing a higher density. The reverse is true for less exposed areas and lower densities. The latent image on your film is negative, meaning the light areas are dark and the dark areas are light.
During your development process your films shadows develop quickly and the highlights build over time. This is an important concept to understand and apply to your development process. This is why you have normal and then contracted (N- 1, N-2) or expanded (N+1, N+2) development times. For example, if you have a very high contrast scene you may want to contract your development time so you don’t lose your highlights. In the reverse if you had a low contrast scene and wanted more contrast, then you could expand your development time. All of this is based on developing your own personal EI rating for your film and then your normal development time. There is no way to establish your N- or N+ times without first having determined normal development.
3.) Film Base – the base of the film must be strong and transparent. Depending on the date of when a film is made may determine the makeup of its base. You will find modern films typically made of cellulose triacetate, acetate or even some form of polyester material. In the olden days the base was actually flammable and could literally catch on fire. You may have an idea now why glass plates were used and how film advanced to the state of using these other materials.
4.) Antihalation Layer – The antihalation layer is made up of a dye that depending on the film may be different colors. The reason it is different colors for different films is because the manufacturer selects a dye color that is least sensitive to the emulsion. It is added to the back of the film base primarily to prevent scratching and curling as well as halation. Halation can happen when light passes through the films emulsion and base and is reflected off the back surface of the base layer back into the emulsion. As you can imagine this is not desirable and those nasty little things we know as halos or bright points are the result. The antihalation layer does its job during the exposure of the emulsion and is washed off during the development process.
5.) Noncurl Coating – The final layer is a non-curling coating of hardened gelatin. This layer is typically about the same thickness as the emulsion layer and as you can guess is applied to the back of the film. When you develop your film the emulsion layer literally swells up and then when your film is dry is shrinks back down. If there wasn’t a noncurl coating you can imagine that your film during the retraction process would do all sorts of bad things that you probably wouldn’t like. Even to this day certain types of film are more prone to curling or cupping than others.
I hope this high level overview about the basic components of your film was helpful to you. So the next time you are pre-soaking your film or pouring out the developer and notice a dye in your output you will know that it was your antihalation layer being washed away. This information may give you some insight why professional photographers use the Zone System to carefully control their negative creation during the exposure and development process.
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