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الصورة الفضية .. اختيار الفيلم .. كيفية التقاط وتطوير الصور بالأبيض والأسود
CHOOSING FILM
THE SILVER IMAGE
Lacking the element of colour , black - and - white films offer a more limited choice of brands and varieties . However , although this alone might make it more straightforward to select the type and make , the sheer versatility of black- and - white film makes it important to understand how it works - probably more so than with colour film . Textural effects that use the film's graininess , and manipulations of speed and contrast depend on your knowing the characteris- tics of different emulsions quite well .
Some idea of the basic film process is an invaluable introduction to these different characteristics . The mechanism is this : large numbers of light - sensitive silver halide crystals embedded in gelatin make up the emulsion . On exposure in the camera , those struck by light undergo a very slight reaction - too small to be visible without the help of a developer . Processing magnifies this change several million times : the action of the developing solution converts each exposed crystal to black metallic silver , while those that have received no light remain transparent . The crystals are so small and numerous that they cannot be seen individually without a microscope ; the pattern of exposed and unex- posed crystals gives a range of tones from almost solid black to nearly clear , with infinite shades of grey in between .
After development , the black silver grains are stable , but the unexposed silver halide crystals are not any light will turn them black too . To prevent this , the film must be ' fixed ' in another solution whch dissolves away the re- maining halides ; and finally , washed and dried .
The result is a negative image , with all the tones reversed : the bright parts of the original scene make the crystals react and turn black . So , the negative is an intermediate stage in producing a normal , positive image . The printing paper used for enlargements is coated with an emulsion that is similar to that of the film , although less sensitive to light . When the negative is exposed onto this in an enlarger , a new negative image is formed . This time , however , it is a nega- tive of a negative and so positive .
A black - and - white negative can vary in only a few ways . The most obvious is in how dark or light it is overall , and this is simply a matter of how much expos- ure the film is given in the camera and how much development in the dark- room . The more of each the film receives , the more of the emulsion will be i darkened .
Three other qualities , however , are closely related to the way the film is made . They are speed , graininess and contrast . We will look at each of these in turn in a moment , but it is important to realise that they are all intimately related . The conventional way of making a film more sensitive to light is for the manufacturer to use bigger crystals of silver halide and to stack them in a thicker layer . This means that , when exposed and developed , the black silver grains form bigger clumps , and this becomes visible as a more grainy texture . You cannot have fast speed without some increase in graininess . Equally , con- trast is related to speed in that slow films have a higher contrast than fast emulsions .
LEFT Because black - and - white films that are more sensitive to light use bigger crystals of silver halide stacked in thick layers , fast emulsions feature very noticeable grain . On the other hand , using slow films ( as in this example ) results in very fine grain and , therefore , higher contrast .
How Black and White Film Works
In black - and - white film , the active element is the emulsion - a thin layer of light - sensitive crystals of silver halide suspended in gelatin . This is spread on a tough , flexible but not stretchable base of cellulose- acetate . Protecting the delicate emulsion layer is a scratch - resistant coating , while under the base is another coating to reduce reflection of light back into the emulsion .
1 When black - and - white film is exposed to the light in the camera individual grains that are struck by light react , but invisibly . The mechanism of this reaction is rather more complicated than might at first be imagined , and is triggered by free independent silver ions and small specks of impurities such as silver sulphide . Some of the silver ions collect together at sites that have been exposed to light , forming a latent image . It is called " latent " because , although real , it still needs the action of a developer to increase it in the order of about ten million times in order to make it visible .
2 Adding developer solution to the exposed film converts those silver halide crystals that contain silver ion traces into black silver metal . At this stage , which must be performed in darkness , the crystals that did not receive any light are still sensitive .
3 The final stage in the process is the removal of the developer and the addition of fixer , which turns the remaining silver halide crystals into salts that can be washed away . When this is complete , the image is stable and the film can be exposed to light without any further changes taking place .
CHOOSING FILM
THE SILVER IMAGE
Lacking the element of colour , black - and - white films offer a more limited choice of brands and varieties . However , although this alone might make it more straightforward to select the type and make , the sheer versatility of black- and - white film makes it important to understand how it works - probably more so than with colour film . Textural effects that use the film's graininess , and manipulations of speed and contrast depend on your knowing the characteris- tics of different emulsions quite well .
Some idea of the basic film process is an invaluable introduction to these different characteristics . The mechanism is this : large numbers of light - sensitive silver halide crystals embedded in gelatin make up the emulsion . On exposure in the camera , those struck by light undergo a very slight reaction - too small to be visible without the help of a developer . Processing magnifies this change several million times : the action of the developing solution converts each exposed crystal to black metallic silver , while those that have received no light remain transparent . The crystals are so small and numerous that they cannot be seen individually without a microscope ; the pattern of exposed and unex- posed crystals gives a range of tones from almost solid black to nearly clear , with infinite shades of grey in between .
After development , the black silver grains are stable , but the unexposed silver halide crystals are not any light will turn them black too . To prevent this , the film must be ' fixed ' in another solution whch dissolves away the re- maining halides ; and finally , washed and dried .
The result is a negative image , with all the tones reversed : the bright parts of the original scene make the crystals react and turn black . So , the negative is an intermediate stage in producing a normal , positive image . The printing paper used for enlargements is coated with an emulsion that is similar to that of the film , although less sensitive to light . When the negative is exposed onto this in an enlarger , a new negative image is formed . This time , however , it is a nega- tive of a negative and so positive .
A black - and - white negative can vary in only a few ways . The most obvious is in how dark or light it is overall , and this is simply a matter of how much expos- ure the film is given in the camera and how much development in the dark- room . The more of each the film receives , the more of the emulsion will be i darkened .
Three other qualities , however , are closely related to the way the film is made . They are speed , graininess and contrast . We will look at each of these in turn in a moment , but it is important to realise that they are all intimately related . The conventional way of making a film more sensitive to light is for the manufacturer to use bigger crystals of silver halide and to stack them in a thicker layer . This means that , when exposed and developed , the black silver grains form bigger clumps , and this becomes visible as a more grainy texture . You cannot have fast speed without some increase in graininess . Equally , con- trast is related to speed in that slow films have a higher contrast than fast emulsions .
LEFT Because black - and - white films that are more sensitive to light use bigger crystals of silver halide stacked in thick layers , fast emulsions feature very noticeable grain . On the other hand , using slow films ( as in this example ) results in very fine grain and , therefore , higher contrast .
How Black and White Film Works
In black - and - white film , the active element is the emulsion - a thin layer of light - sensitive crystals of silver halide suspended in gelatin . This is spread on a tough , flexible but not stretchable base of cellulose- acetate . Protecting the delicate emulsion layer is a scratch - resistant coating , while under the base is another coating to reduce reflection of light back into the emulsion .
1 When black - and - white film is exposed to the light in the camera individual grains that are struck by light react , but invisibly . The mechanism of this reaction is rather more complicated than might at first be imagined , and is triggered by free independent silver ions and small specks of impurities such as silver sulphide . Some of the silver ions collect together at sites that have been exposed to light , forming a latent image . It is called " latent " because , although real , it still needs the action of a developer to increase it in the order of about ten million times in order to make it visible .
2 Adding developer solution to the exposed film converts those silver halide crystals that contain silver ion traces into black silver metal . At this stage , which must be performed in darkness , the crystals that did not receive any light are still sensitive .
3 The final stage in the process is the removal of the developer and the addition of fixer , which turns the remaining silver halide crystals into salts that can be washed away . When this is complete , the image is stable and the film can be exposed to light without any further changes taking place .
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