The Concept

A typical sheet of film being struck by light

Early cameras were not able to capture a picture, only project it. The idea of making an image "permanent" has been around ever since the earliest paintings, but special film was required to make photographs permanent. Since the image projected by a camera is simply light, film needed to be designed to react to the amount of light falling on it. Fortunately, silver halides fit the description. When hit by a ray of light, the bond between the silver and the halide is broken, causing the silver to reduce to pure silver metal. As long as the film is only exposed to light coming from the camera, it will faithfully record the image projected, creating a photograph. (Note that we will only be discussing grayscale film.)

The Logistics

Modern film generally consists of a sheet of plastic coated with a layer silver halide crystals. These crystals are suspended in a gelatinous layer that is on top of the paper backing. This is the simplest type of film, one of the earliest ones to be developed. Since it is sensitive to any and all light, it must be stored in a dark location in order to prevent it from being "exposed" before its time.

This may look gray, but it's actually black dots

One problem with this design is that there is no "sensitivity" of the film. That is, the film is black and white, without shades of gray: every crystal is either exposed or not. To account for this, the film actually has crystals of multiple sizes placed throughout the gelatinous medium (almost homogenously). The different sized crystals require different amounts of light in order to break. The larger the crystal, the more energy that you need to react it. Thus, depending on the intensity of the light, a different amount of crystals are activated. Since the crystals are distributed evenly and tightly packed, an area with fewer affected crystals will appear gray even though it is actually a bunch of closely packed black dots.

Fact Check! Isn't silver white? Yes it is. In fact, silver tends to reflect light. When light hits a silver halide, it reduces it to a pure silver crystal. And these effectively act as the "pixels" of the film, appearing black and grey. So the question is, if these are also silver crystals, why aren't they white? Well, the reason that silver crystals are white in the first place is because they reflect almost all of the light sent towards it. However, this light is only reflected back at the viewer's eye if the crystals are in a relatively "normal" shape. Actually, the silver crystals are, in reality, nothing at all like the ones pictured in the image above. Rather, they are weirdly shaped, rather spidery particles. Instead of reflecting light back at the viewer like a normal crystal, these crystals tend to reflect the light inwards, away from the viewer, making it seem black because no light is coming back. Since silver is such a great reflector, it reflects almost all of the light away from the viewer; nothing will be left to see!


So far, we've actually been deceiving you somewhat. The diagram above makes it seem as if the crystals turn black as soon as they is exposed. While it is true that the chemical reactions occur as soon as light hits the film, it is hardly evident, as the resulting (pure) silver crystals are too small to be seen by the naked human eye. In order to actually see what's on a piece of filmpaper, it is neccessary to develop it. The development process is generally split into four steps:

Step 1: Water Bath

Generally, the photo paper is first soaked in plain water to grow the gelatinous layer. This is not strictly necessary, but it makes it easier for the development chemicals to permeate throughout the entire photo evenly.

Step 2: Developer

When light hits the film, it actually only affects a very small piece of the entire silver halide crystal. The developer is a chemical used to expand the reduced silver crystals: that is, to cause the crystals surrounding the reduced crystals to reduce as well. Therefore, what the developer does is simply make the photo darker.

The chemicals that are used for this process only function properly in a basic enviornment (pH > 7), so they are almost always mixed into a basic solution. This property is also the basis for the next step of the development process.

Step 3: Stopper

Developer chemical is a tricky little devil. On the one hand, it's a necessity for the development process: you can't see your pictures without it. However, since all it does is darken the picture, if you leave the photo in there too long, it comes out completely black. The hardest part of developing a photo is leaving in the developer for the right amount of time.

After the picture has been in the developer for a long enough time, it is necessary to put it into a stopper to stop the silver halide crystals from reducing further. The stopper takes advantage of the fact that the developer chemicals only work in alkaline solution to stop them: it is just an acid solution. Very simply: the developer chemicals stop working after they are placed into acidic solution. The stopper also "deactivates" the remaining silver halide crystals in order to prevent the photo from being re-exposed.

Step 4: Water Bath

After the process is complete and the picture is visible, it is still coated with chemicals that can sometimes be rather nasty to handle. Thus, the photo is put into another water bath in order to clean off the excess chemicals. Voilà! You now have a fully developed picture.

Remember, however, that all of these processes have to be done in a "dark room," so that the photo is not exposed to any light during development.