Film is magic …

… but, should a magician reveal their tricks? Or how does film work, what is actually going on.

Personally I have shot film, developed and printed photos since the 1980’s. Yet I still love the magic of opening the developing tank and checking that there is an image on my film after developing. I still love setting up the negative in the enlarger exposing the paper and then watching as the image reveals itself (as if by magic) in the developer tray. But what is actually going on there in the chemicals and emulsions to make the magic happen?

When we run a workshop at District Darkroom, the conversation will often come around to what is actually going on there to actually develop your film or print. So I thought that I would write a description which is a little bit more detailed than the super simplified version that I try to keep to in the workshop.

Because it is the most simple and the easiest to explain, this post will stick to what is happening when we develop black and white materials. This is a relative concept though because it is only simple in relation to colour film and printing. The description is what will be simplified, there is actually a lot more going on behind the scenes at Kodak and Ilford and Ferrania (or wherever your favourite light sensitive product is made).

How are film and paper made?

The simple version of Black and white film and paper can be seen as coating a very thin (5-25 micron depending on emulsion speed) light sensitive emulsion onto a base material. In the case of film, the base material is transparent, in the case of paper the base is reflective. In both cases, the base makes the emulsion dimensionally stable but it is still flexible so the it can be packaged into roll form. The emulsion is made up of grains or crystals of light sensitive silver (silver halides, usually silver bromide) evenly dispersed in a gelatin mixture. When the film or paper are being manufactured, the emulsion is in a viscose liquid state and in a part of the factory called the coating track, a thin, even layer of the emulsion is coated onto the base. In the case of film, this process will happen in total darkness, in the case of black and white paper appropriate safe lighting can be used if necessary. The coating process happens on wide rolls of the base material and once the emulsion has dried the wide rolls are then cut down into smaller widths to suit the product being made.

Figure one: a cross section of a theoretical, simple single layer film. Light sensitive silver bromide crystals are suspended in gelatin in the emulsion layer. The base layer is typically acetate in film, paper with a plastic coating in the case of resin coated printing paper and thick paper in the case of fibre based printing paper.

In actuality, there are more layers to the emulsion to add features and characteristics to the final product such as anti halation layers in film and high and low contrast layers in variable contrast papers but these are subjects for future articles.

The figure above shows a cross sectional view of our theoretical film. The film base is much thicker than the emulsion which has been coated on it and the emulsion shows the individual silver grains dispersed in the gelatin. Gelatin is an important component because it holds the grains of silver in place, it is stable over long periods of time, it is flexible to allow roll film to be a practical option and it is permeable to liquids allowing developing chemicals to reach the individual silver grains.

Exposure

So let’s have a little look at the magic. When film is in your camera and you press the shutter button to take the picture, little atomic level reactions are happening in each of the silver grains. Way down at the atom level, the light that hits the silver halide crystal energises the electrons in atoms that make up the grain, this enables them to move to an outer conductance band of the atom. The more light energy that hits a particular grain, the more atoms will have electrons will be able to move into the conductance band. Once electrons are in the outer band they are free to react with other compounds. No further reaction has happened yet, that will come in the developing stage, but all these silver grains that are ready to do something have formed a latent image of what you have photographed. The latent image is there down at the atomic level but not visible yet. The film is of course still sensitive to light and if you were to allow more light to hit the emulsion at this stage, more electrons would be excited and more latent image would form. This might be deliberate, think double exposure, or accidental, think opening the camera back before the film has been rewound. Clearly, we should get that film developed.

Figure two: The latent image being formed. The darker grains indicate areas where electrons have been excited by the light, there is not any actual visible change, the reaction is down at the atom level.

The figure above shows various levels of light hitting different areas of the emulsion. The silver grains which have formed a latent image have been changed to a darker tone, in reality there is no visible change to the silver grain, this is just to illustrate the grains that will react with developer in the next stage.

Developing

Once we have finished shooting our theoretical film, the next step is to develop the images we have taken and to make the film not sensitive to light any more so that we can look at our photos. Once again, this is the really simplified version so we are not going into exactly what goes into a developer or a fixer but just having a brief look at what is going on in the mixtures. All developers will have a developing agent, this reacts with the latent image to form black metallic silver particles. So in the exposure step you will remember that the more exposure that a grain of silver received, the more electrons moved into the outer conductance bands. These electrons are what will now react with the developing agent to change the silver bromide to metallic silver. If there was a lot of exposure on a particular silver grain, Lots of electrons will be in the conductance bands of the atoms in a particular grain so all of that grain will be converted to metallic silver. If a grain only received a small amount of exposure, only some atoms in the grain will have excited electrons in the conductance band and so only part of that grain forms metallic silver. What you end up with are grains of black silver that are sized in proportion to the amount of exposure that each grain received in the camera.

Figure three: Developer has reacted with the latent image.The silver grains which only received a slight exposure to light will react with the developing agent and partially change to black metallic silver. Grains which received a medium level of exposure will convert to metallic silver a bit more, and grains which received a lot of exposure will fully convert to metallic silver.

At the end of the developing time though the film is still light sensitive. All the silver bromide which has not reacted with developing agent is still sitting there in the emulsion. Two more chemical steps are needed. Stop bath, this has a simple job, to stop development. If you remember chemistry from high school, the way to neutralise an alkali is to add an acid. Developing agents are typically slightly alkaline so stop bath is usually slightly acidic to stop development. You can also use water as a stop bath, the action is slightly different as you are essentially diluting the developing agent to to such an extent that it no longer has the ability to do any developing.

The final chemical step is fixing. The fixer dissolves the silver halides which have not reacted and changed to metallic silver so that the film is not sensitive to light any more. Fixers will usually be largely made up of Sodium or Ammonium Thiosulphate and they will react with the silver bromide but not the metallic silver so when the fixer has done its job you are left with your negative image. That is the simple description of how this magic is done.

Figure four: During the fixing stage, the light sensitive silver halide is dissolved by the fixer and it comes out of the emulsion, remember earlier when we said that gelatin is permeable to liquids? Well now that is another little bit of magic happening there. You are left with the black metallic silver grains forming an negative image. Tiny grains in the slight exposure areas and large grains in the areas which received a lot of exposure.

Wash your film really well (you want to remove all the fixer because it is not long term stable like black metallic silver and gelatin) and you are ready to have a look at how you went.