How Photographic Film Works
Some of you may know this. Some of you may not. But here’s something interesting: nowadays, cameras are everywhere, as in, just about everyone has one in their pocket. Sure, digital cameras are just counting photons hitting some silicon. But, before that? We had film. Film that recorded photos physically with chemical changes. And, I personally find that the science behind that is rather fascinating.
I’m only covering standard photographic film here, not slide film. Normal film results in a negative, where darker parts of the image, and vice-versa. Colors are also completely inverted. Slide film doesn’t do this. Once correctly developed, slide film is a little miniature piece of material that exactly shows what was captured on it (though, the process to do this is slightly more complicated, hence why I’m not describing it). However, there’s still two categories: black and white, and color. And while they work off the same basic idea, the actual implementations are a little different.
All film starts with the film base, which has historically been a few things, but you’re almost always going to see cellulose triacetate as the base of choice. Onto this, we apply what’s called the ’emulsion,’ which is some silver halide dispersed into… gelatin. Yes, gelatin. Between the emulsion and the film base is what’s called an antihalation layer, which prevents light from passing through the emulsion, bouncing back off the film base, and causing more reactions than it should have. See, silver halide is light sensitive. When a photon hits a silver halide crystal, it can cause a speck (like, a single atom) of metallic silver to form. A single Crystal needs about 4 atoms to be developable into a usable image. This is why film that’s more sensitive, called ‘faster’ film, has larger film grain: with larger silver halide crystals, the chance that enough photons hit that single crystal increase, since its area has increased.
A process called development is what turns the few atoms of silver, called the latent image, into the perceptible negative that can be used. The actual process used, and the specifics of how the silver halide is used, depend on the type of film, like I said before.
Black and White Film
B/W film just has one emulsion layer, and has nothing special about it. The developer for it is a reducing agent, meaning that it will, if I remember my chemistry, donate (an) electron(s) do the silver halide, thus causing it to form as metallic silver. This can only happen with some silver to seed the process, hence the need for at least 4 atoms present to begin with. The more silver atoms present, meaning the more light hit that grain, the quicker it’ll be filled with silver, and the darker that spot will be. Yes, B/W film looks dark because you’re seeing light getting trapped between particles of silver suspended in a layer of gelatin. B/W developers are basic (high pH), and, as one might guess, this process is time sensitive. Too much time in the developer and it’ll be too dark, not enough time, and it’ll be pretty faded. While you can use a ‘stop bath’ (just a weak acid) to stop the developer, you can also just… use this new piece of tech called water just as well. Developers also usually have some chemical like potassium bromide in them, to prevent ‘fogging’ the image, basically a slight modification to the development speed to leave a bit more contrast between light and dark areas.
After the developer is the fixer, which removes the remaining ‘undeveloped’ silver halide from the emulsion, thus making the film insensitive to light. The active ingredient in most modern fixers is ammonium thiosulfate ((NH4)2S2O3), though some older fixers would use sodium thiosulfate (NaS2O3) instead, also called ‘hypo.’ While color film also needs fixer much the same, it’s slightly different. You’ll see… right now!
Color Film
Color films generally have multiple emulsion layers, each sensitized to a certain wavelength (color) of light. A classic example involves a stack-up of a blue-sensitive top layer, then a yellow filter to remove all the blue light, then a green layer, then a red layer. Since all silver-based emulsions will be sensitive to blue, the use of the yellow filter is used to block blue from interfering with the other layers. Within those emulsion layers are more than just solver particles. There’s also ‘dye couplers’ mixed in, which are just waiting for some oxidized developer to activate them. Just about every color film uses the C-41 development process, which is it’s own fun little dance. First, you need a color developer. p-Phenylenediamine, or, C6H4(NH2)2, is the common chemical, which develops the silver. Since it reduces the silver, that means it also oxidizes the developer. The oxidized developer will inevitably come into contact with some nearby dye coupler, which will react and form the appropriately colored dye right there in the gelatin.
This is immediately followed by a bleach, which converts all the silver back into silver halide, so the fixer can remove it all. As for fixers, nothing special is needed. Ammonium thiosulfate has no reaction with the resulting dyes in the film. Silver plays no part in the formation of the final image in color film, it really only exists as a known chemical process where we can measure the number of photons that struck a certain area.
It’s interesting to think about the fact that with B/W film, you’re literally seeing the result of silver blocking light. But regardless of which type of film, the process of using photons to create just a few atoms of silver that can then be used to create an entire visible image with a little chemistry is just… I really have to figure out how this came into existence. It, like a lot of things that I deal with, really sounds like it shouldn’t work when you first explain it. Like yeah I’ll find a way to take a quantity of the smallest unit of matter in the universe so small I can count on my one hand, and use that to reproduce an entire image, sometimes with color is just astounding to me. But you know what? It works. It works so well, it was the standard way to record photographs for quite some time (until digital cameras came along with their fancy photon-counting silicon rectangles…)