I’ve been interested in photography ever since I bought my first camera as a kid–an all-plastic (including the lens) special that cost all of $1. Over the next couple of years, with that camera and its sequel, a $10 Instamatic, I shot lots of pictures of friends, scenery, and my cat (LOTS of pictures of my cat). Then in junior high I discovered photography’s secret world, hidden in dark rooms, lit by ghastly red lights. Cue the eerie organ music…
The fundamental scientific principle of photography is that light falling on the grains of certain insoluble silver salts (silver chloride, bromide or iodide) produces invisible changes in those grains, apparently creating tiny flecks of black silver inside them. Chemical solutions can then be applied to the affected grains to turn them entirely to black silver.
Johann Heinrich Schulze discovered this in 1727, but a century passed before various people succeeded in using his discovery to record an image. They combined it with another long-established device, the camera obscura, in which light passing through a small hole or lens in one wall of a dark room or box projects an inverted image on the opposite wall. The new process, called photography (from the Greek “photos,” light, and “graphein,” to draw) was first made public in France in 1839.
Modern cameras, for all their electronic thingamabobs, are really just elaborate camera obscuras (cameras obscura?). When the shutter is opened, light passes through the lens to form an inverted image on the film inside the camera. Then chemistry takes over.
Black-and-white film consists of a transparent base covered with a thin layer of gelatin in which is suspended tiny mixed crystals of silver bromide and silver iodide. (The gelatin not only holds them in place but increases their sensitivity to light.) Special dyes are also added to this mixture, or “emulsion,” to make the crystals more sensitive to the red end of the spectrum.
Colour film has three layers in the emulsion, sensitive to red, green and blue light, respectively. Any other colour can be made from these “primary” colours.
The crystals can be of various sizes, depending on the film. Generally, the larger the grains, the more sensitive (or “faster”) the film, probably because large grains intercept more light. This graininess shows up in the final print, though, so photographers must balance the need for fast film with the need for fine detail.
When an image is cast on the film, many crystals are affected by it in areas receiving lots of light many crystals are affected, while fewer are affected in the darker areas. This creates what is called a “latent” image on the film: you can’t see it, but it’s there.
In black-and-white development, the exposed film is transferred in darkness into a developing solution, which causes those grains affected by light to turn wholly into black silver. The film is then rinsed in a stop bath of acid or water to stop development and remove the developer, and finally “fixed” in a solution that dissolves and removes the unchanged silver salts, making the film no longer sensitive to light. The resulting image is a negative, black where light struck the film and transparent where it didn’t.
To create a print, light is shone through this negative onto paper that has been covered with an emulsion similar to that on the film, although less sensitive to light. The paper goes through the same development steps as the film did, and produces a negative image of the negative–in other words, a positive, which accurately reflects the original.
In colour film, the three emulsion layers contain substances called “dye intermediates.” During development, these are converted into dyes that stain each of the three silver images in a colour “complementary” to the colour of the light that formed it–in other words, blue areas turn yellow (which is white light minus blue), green areas turn magenta (minus-green), and red areas turn cyan (minus-red). The silver itself is bleached and removed, leaving only the transparent dyes.
Colour print paper has the same three emulsion layers as film and is developed in essentially the same way. Shining light through the colour negative casts an image on the paper made up of the complementary colours of the original; developing the paper creates an image with the colours complementary to the complementaries: the original primaries.
Colour slide film works a bit differently. It also has three layers sensitive to the primary colours, but it’s developed twice. The first time the three layers aren’t stained; this produces three black and white images, each representing a different colour, overlaid on each other. The film is exposed again to light from a lamp, which triggers changes in those film grains that weren’t affected by the initial exposure.
In the second development, dye intermediates colour the newly affected grains (but not the ones originally affected) in their complementary colours: the red-sensitive layer becomes cyan, the green-sensitive layer magenta, and the blue-sensitive layer yellow. Then both sets of silver images are bleached and washed away, leaving only the dyes. When you shine white light through the slide, the minus-green and minus-blue dyes remove green and blue from the white light, leaving red, the minus-green and minus-red dyes leave blue, and the minus-blue and minus-red dyes leave green: the original colours.
For most of us, the greatest achievement of photography is its ability to capture memories. But next time you look at that beloved photo of your sweetheart of 1974, spare a thought for the dark–or, at least, dark-room–scientific secrets that made it possible.
