Part 1: How Film Works
Film photography is magic. You take a photo, but you can’t see it right away which in photography is called a Latent Image. The image is captured on the roll of film inside your camera but the film needs to be developed in a darkroom first in order to see the actual exposure you took.
Before digital cameras and smartphones, film cameras were the only way to take pictures. These, like digital cameras come in various sizes and use a lot of different film types, the most common two today are the 35mm or the 120mm (Medium Format) film, which comes in different types, like Kodak’s Kodak Gold 200 Colour Film or the Ilford HP5 400 B&W Film.
But how does Film work?
The actual film is built up of several layers which in union creates the Film roll we all use. To better understand this we’ll start with B&W as it has less components in it.
Looking at this diagram you can see a basic representation of the different layers that a B&W Film is made up of.
- The first layer is the AntiScratch layer which is made up of Naked Gelatin. This layer’s main purpose is to protect the Emulsion from suffering physical Damage.
- The next layer down is the Emulsion. This layer is a mixture of gelatin and Silver Halide Crystals. These crystals are what makes the film light-sensitive which makes it able to record exposures. They are scattered in a gelatin substrate to provide a buffered environment as well as stabilising some chemical reactions. The Light sensitive Silver Halides used in film photography are mostly based on Chlorine [Cl] and Bromide [Br]. Silver Bromide [AgBr] is often used as well.
- The third Layer is the Antihalation Layer which is a layer made to absorb most of the light that would otherwise reflect back up through the emulsion, scattering light and reducing the sharpness of the image.
- The fourth layer is the film base. This acts as a support medium for the photosensitive Emulsion to lie on top.
- The last Layer is the Antistatic Backing Layer which prevents any static that may build up and damage the film.
How the Image is formed
To better understand how the exposing process is done let’s look at a specific scene of a lit lamp in a dark space.
Once the film is exposed to light the latent image is similar to the illustration below.
The dark part of the scene matches the unexposed Silver Halides since no light struck them. The light part of the scene, which is the part where the lamp is in the frame, matches crystals dotted by a black point (representing the development seed) on the film emulsion.
Then, the latent image is sort of intensified by the developer agent action.
All the exposed Silver Halide crystals will be totally transformed into metallic Silver. The latent image is therefore revealed or developed and its macroscopic density is now sufficient to be of some use.
Now the emulsion must be made non sensitive to light so that we produce what we know as the negative.
The metallic Silver is already desensitised, but the Silver Halide crystals which were not exposed (the one corresponding to the dark part of the scene) are still light sensitive. The most obvious thing to do is to get rid of those i.e. flush them out of the emulsion. This is the function of the fixer agent, it dissolves the Silver Halides into a solution. This means, the crystals now being soluble they can be flushed out with the fixer agent.
In this last illustration above, we can see that only the exposed parts are covered with metallic Silver. The rest of the image which remained unexposed has been thoroughly cleaned by the fixer agent removing all Silver Halides that could not turn into metallic silver.
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