The Science of Photography - Chemical Image Processing
Once an image has been formed in a
controlled manner, see Forming
an Image, a convenient method of recording and viewing it is required.
Before photography was invented artists would use the Camera Obscura to
assist them in drawing the image. Photography introduced chemical methods
for recording and printing the image. Now images can also be recorded electronically
in digital format and output via printers and monitors.
In 1727 Johann Schulze discovered the light sensitivity of silver dissolved in nitric acid and in the early nineteenth century the sensitivity of silver to light was found to increase when combined with a halogen element, such as chlorine, bromine or iodine.
The resulting silver halides are developable so the effects of light on the halide crystals can be amplified about a thousand million times by using a developing agent.
Silver halide crystals need to be held on a support, such as plastic or glass, by a binder. The binder needs to hold the crystals evenly in place but must also allow the development agents to react with the silver halides.
A wide range of binders have been used, from albumen (egg white) to collodion. However in 1871, over 30 years after the birth of Photography, Dr. Maddox writing in the ‘British Journal Of Photography’ suggested gelatin. Gelatin, derived from a protein found in animal bones, is still in use today. Gelatin has remarkable properties making it an excellent binding agent. The suspension of silver halide crystals in a binding agent is called an emulsion.
The light sensitive emulsion layer of a film contains silver halide crystals or grains. Grain size largely determines the resolving power, contrast and speed of the film.
The resolving power increases as the grain size decreases. Fine grained and thinly coated emulsions are best able to resolve fine detail. However fine grained emulsions are "slow".
The speed of the film is a measure of its light sensitivity, the faster the film the more sensitive it is to light, therefore needing less exposure than a slower film. Generally faster films have larger grain sizes and thicker emulsion layers. All modern films quote a standard ISO speed rating, the higher the ISO rating the faster the film.
The emulsion contrast, that is the range of grey tones it is capable of forming between black and clear emulsion, is largely determined by the distribution of grain sizes. An emulsion that only contains grains of equal size will be high contrast, forming a negative with only black or clear areas. An emulsion of mixed size grains is capable of forming a wide range of greys.
Move your cursor over the links below to see different levels of contrast.
Emulsions are only able to record colour as a tone of grey. Ideally the tone of grey used for each colour should match the colour sensitivity of your eye, although it maybe difficult for you to imagine grey tones instead of the colours you actually see.
Due to the construction of the human eye we see "yellow-green" as the brightest colour, shorter and longer wavelengths of the visible spectrum appear increasingly darker.
However all silver halides are naturally sensitive to "blue" light, making blue skies white and green grass and red flowers very dark.
In the late nineteenth century Dr. Vogel discovered the effects of adding diluted dyes to the emulsions. Now dyes can be added to make emulsions sensitive to red and green, these films are called 'panchromatic', meaning sensitive to all colours.
The diagram below show the differences between blue sensitive and panchromatic emulsions. Move the cursor over the links to see changes in the resulting image.
The negative produced from the film development process records light areas, the highlights, of the subject as dark, whereas the shadows are not recorded and simply become clear emulsion.
In order to provide a positive print from a negative the process of chemical image recording is repeated, normally using photographic paper. Photographic paper, like film, is coated with a light sensitive silver halide emulsion.
Light is passed through the negative onto the paper, either by placing the negative directly on the paper to form a contact print or by projecting the negative image onto the paper by an enlarger.
Printing paper requires slightly different characteristics to photographic film. For example "speed" is less of an issue as longer exposure times can be given in the controlled darkroom conditions. However image contrast needs to be controlled and different print surfaces, for example glossy or matt, are often required.
All these characteristics,
and many more, are available with the wide range of black and white photographic
printing material currently on the market today.
Colour film consists of three layers of emulsion, each layer basically the same as black and white film but only sensitive to one colour, blue, green or red.
Reversal image creation follows these steps :
The diagrams below show the
various stages of reversal image creation, move your cursor over the links
to see the different thumbnails images.
Like colour reversal slide film, colour negatives consist of three layers of emulsion, each layer basically the same as black and white film but only sensitive to one colour, blue, green or red. However, unlike slide film, the colour negatives are only developed once, the positive colours are created during the print development process.
Colour negatives are created by the following steps :
A colour positive can be created by printing the colour negative onto a three layer emulsion, similar to the negative film but using a paper support.
Colour prints are created by the following steps, almost identical to those for creating a colour negative :
The diagrams below show the
steps required to make a colour print from a negative, move your cursor
over the links to see the various stages :
There are many processes between capturing and viewing the image. Each process provides the creative photographer with the opportunity to adjust and manipulate the image.
The choice and use of chemical photographic material contributes to the appearance of the final image. However the greatest opportunity for manipulation is provided by the negative to positive printing process.
Although commercial colour printing machines are designed to simply reproduce the original negative, hand printing black and white negatives can be a very creative process. The printer is able to crop the original image and adjust exposure times, dodging and burning-in specific parts of the image. The printer can even merge and superimposed multiple negatives into a single print.
Image manipulation has been occurring since the birth of photography, however it has always been an area of dispute. Some people insist that any manipulation is a violation of photographic integrity, others argue that creative expression should not be suppressed.
The argument has never been resolved and continues even more strongly with the growth of digital photography.
Next in the Science of Photography : Digital Image Processing