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المواد الحساسة للضوء .. مبادئ التصوير الفوتوغرافي .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
Light - sensitive materials
There are two essential stages to taking a photograph . The first is forming an image of a subject , the second is recording the image permanently . You have seen how the camera forms an image using a lens and a small aperture to focus the reflected light . This image is simply a pattern of brighter and dimmer light . corresponding to light and dark areas of the subject . On pages 16-17 you saw that light is a form of energy . Energy can cause changes in materials , and it is the changes caused by light which record the image . If a material which is changed by light is exposed to a light image , it will change more where the image is bright , less where it is dim . In this way it may slowly take on an imprint of the light and dark area of the subject .
It is easy to test this out quite simply without using photographic materials at all . Leaves use the energy of sunlight to make chemicals essential to their growth . One of these chemicals is the green pigment , chlorophyll . If you tape a black shape on to a living green leaf , and leave it there for several weeks , it will block out the effects of the sunlight . As a result , the covered area will turn a paler green than the rest of the leaf . This is because the transfer of energy from light to chlorophyll - the green pigment - has been halted .
Many other familiar materials will change when exposed to light . A newspaper in the sun with an opaque object on top will turn yellow except where it was covered . A lawn will leave the pale shape of a water hose imprinted on the darker grass . Tanned skin will show the outline of your wristwatch .
- By the early eighteenth century , it was known that some chemicals particularly silver salts darkened rapidly when exposed to light . A hundred years later , in the 1820s , attempts were made to use these silver chemicals to record the image formed in the " camera obscura " . A layer of these light - sensitive silver salts ( halides ) was coated on to a flat surface , and placed where light from an image fell .
How silver salts record an image :
You can see yourself what the result was . Take a piece of modern photographic film or printing paper ( which is also coated with silver salts ) , and leave it out in sunlight with a small opaque object , such as a bunch of keys , placed on top . After about twenty minutes , the parts unprotected by the keys will have turned a dark purplish gray- the creamy silver salts will be changing into black , finely divided metallic silver . If you leave the film or paper out in overcast daylight , it will take longer to darken . This shows that light energy affects the silver salts cumulatively - either by short exposure to bright intensity , or long exposure to dim light .
When you lift the keys off the light - sensitive surface , their shape is visible as a pale silhouette against a dark surround . In terms of light and dark , this image is the opposite of the original , dark keys on a white surround . The image formed is therefore said to be a negative image .
Later in the book you will see how you can use this simple technique , in the controlled conditions of a darkroom , to make photograms ( see p . 132 ) . By arranging objects directly on a sheet of photographic printing paper or film , and then exposing it to light for a specific time , you will create a negative image of the objects in black , white and gray . But you cannot just walk out of the darkroom with your carefully exposed image and expect it to remain unaffected by any more exposure to light . Light - sensitive materials have to be processed to make images permanent not sensitive to any further exposure to light . The development of these processes is an important part of the history of photography .
In the early days of photography , there were three main problems to be solved in recording the camera image : how to make silver salts react to very short exposures - even fractions of seconds - when presented with the rather dim image formed by the lens : how to prevent the picture from continuing to darken each time it was brought out into the light to be examined ; and how to turn the incorrect , negative toned image formed by the action of the light on the salts into a correct , positive toned one .
The modern photographic process :
The light - sensitive silver - based materials used in modern films and photographic papers solve all these problems . You expose the film to light only long enough to start the darkening process . At this stage , the change in the material is not visible to the eye : but wherever light has reached the pale silver salts they have begun to turn to black silver . The film is kept in the dark , and treated with a chemical solution which develops and accelerates this change until a strong visible black silver image appears . Still in darkness , it is then treated with another kind of chemical - a fixer . This desensitizes the remaining white silver salts unaffected by light , allowing them to dissolve away by washing . You are left with a stable negative image , which can then be safely exposed to light without any further change occurring .
Turning the negative into a recognizable positive picture is quite simple . If you shine a beam of light through the negative . you can project the image on to another light - sensitive material -a sheet of photographic paper . This time , the dark areas of the negative will produce little response in the silver salts , while the light areas will produce a strong dark image . So after the paper has been developed and fixed you have a positive image a picture with light and dark parts of the original subject correctly reproduced . Mid - tones in the subject appear as a range of grays . because the light has only partially darkened the silver salts . This negative / positive process is very useful you can enlarge the image , alter its tones , and take as many copies from a single negative as you like .
One further problem remained . Silver salts are sensitive only to blue light . So they respond to blue and white light ( which contains blue light ) , but " see " red and green , the other two . primaries , as " no light " - that is , indistinguishable from black . Not until the end of the nineteenth century was it discovered that adding traces of dyes which absorb red and green could create a more normal , gray tone response to these colors . The result was panchromatic " film for black and white photography . The same discovery later led to color photography .
Pigments that absorb certain color wavelengths are also present in the human eye - in the cells sensitive to color which line the retina . In fact there are many features in common between how your eyes see , and how cameras and film work . But this does not mean your photographs will record the world just as your eyes see it and your brain interprets it . There are several basic differences - and these form the third basic element of photography , looked at on the next two pages .
Sensitivity to light :
Vegetation is light - sensitive Leaves use energy from sunlight to " photosynthesize " ( manufacture ) essential chemicals including the green pigment , chlorophyll . If you tape a black shape on to a living green leaf for several weeks it will leave a mark which is paler green .
Printing with sunlight When exposed to bright sunlight , some dyes and pigments begin to bleach , and cheap papers often discolor . You can see this by placing an object , such as a leaf , on a piece of newspaper in the sun . The cheap paper will become yellow except where protected from the action of the light by the object .
Sunburn Skin is sensitive to the sun's rays . Both visible and ultra - violet radiation stimulate it to produce a dark
pigment , visible as tanning . Shapes on the skin protected from the sun . such as a wristwatch , are clearly visible as lighter areas .
Exposing photographic paper to light Most salts of silver darken under the action of light . They break down into finely divided dark metallic silver . even without any processing . You can see this by taking a sheet of photographic printing paper from its packet in the dark , and positioning an opaque object such as a bunch of keys on its sensitive surface . Take this out into sunlight and let it stand .
Gradually the light - sensitive salts turn gray or purple ( a rich black is never reached ) . When you remove the keys , their shape remains visible on the paper as a white , negative image . But the white shape is not permanent and begins to darken once it is no longer hidden from light . Instead of using a long exposure to imprint the image direct , you can give a brief exposure under weaker light and then amplify the image to a rich black using developer chemicals . The result is a bold , permanent negative image .
Obtaining a positive from a negative image
A simple shape like a bunch of keys may be acceptable as a negative image , but pictures taken in the camera would look very odd if left with all their light and dark areas tonally reversed . To convert the negative into a positive , you simply print in on to another sheet of light sensitive material . Take the paper negative of the keys , top left , and press it face down on to photographic printing paper . Then direct a lamp through it for about a minute . After processing , the new sheet of paper will carry a " negative of a negative " - a positive image , left .
Light - sensitive materials
There are two essential stages to taking a photograph . The first is forming an image of a subject , the second is recording the image permanently . You have seen how the camera forms an image using a lens and a small aperture to focus the reflected light . This image is simply a pattern of brighter and dimmer light . corresponding to light and dark areas of the subject . On pages 16-17 you saw that light is a form of energy . Energy can cause changes in materials , and it is the changes caused by light which record the image . If a material which is changed by light is exposed to a light image , it will change more where the image is bright , less where it is dim . In this way it may slowly take on an imprint of the light and dark area of the subject .
It is easy to test this out quite simply without using photographic materials at all . Leaves use the energy of sunlight to make chemicals essential to their growth . One of these chemicals is the green pigment , chlorophyll . If you tape a black shape on to a living green leaf , and leave it there for several weeks , it will block out the effects of the sunlight . As a result , the covered area will turn a paler green than the rest of the leaf . This is because the transfer of energy from light to chlorophyll - the green pigment - has been halted .
Many other familiar materials will change when exposed to light . A newspaper in the sun with an opaque object on top will turn yellow except where it was covered . A lawn will leave the pale shape of a water hose imprinted on the darker grass . Tanned skin will show the outline of your wristwatch .
- By the early eighteenth century , it was known that some chemicals particularly silver salts darkened rapidly when exposed to light . A hundred years later , in the 1820s , attempts were made to use these silver chemicals to record the image formed in the " camera obscura " . A layer of these light - sensitive silver salts ( halides ) was coated on to a flat surface , and placed where light from an image fell .
How silver salts record an image :
You can see yourself what the result was . Take a piece of modern photographic film or printing paper ( which is also coated with silver salts ) , and leave it out in sunlight with a small opaque object , such as a bunch of keys , placed on top . After about twenty minutes , the parts unprotected by the keys will have turned a dark purplish gray- the creamy silver salts will be changing into black , finely divided metallic silver . If you leave the film or paper out in overcast daylight , it will take longer to darken . This shows that light energy affects the silver salts cumulatively - either by short exposure to bright intensity , or long exposure to dim light .
When you lift the keys off the light - sensitive surface , their shape is visible as a pale silhouette against a dark surround . In terms of light and dark , this image is the opposite of the original , dark keys on a white surround . The image formed is therefore said to be a negative image .
Later in the book you will see how you can use this simple technique , in the controlled conditions of a darkroom , to make photograms ( see p . 132 ) . By arranging objects directly on a sheet of photographic printing paper or film , and then exposing it to light for a specific time , you will create a negative image of the objects in black , white and gray . But you cannot just walk out of the darkroom with your carefully exposed image and expect it to remain unaffected by any more exposure to light . Light - sensitive materials have to be processed to make images permanent not sensitive to any further exposure to light . The development of these processes is an important part of the history of photography .
In the early days of photography , there were three main problems to be solved in recording the camera image : how to make silver salts react to very short exposures - even fractions of seconds - when presented with the rather dim image formed by the lens : how to prevent the picture from continuing to darken each time it was brought out into the light to be examined ; and how to turn the incorrect , negative toned image formed by the action of the light on the salts into a correct , positive toned one .
The modern photographic process :
The light - sensitive silver - based materials used in modern films and photographic papers solve all these problems . You expose the film to light only long enough to start the darkening process . At this stage , the change in the material is not visible to the eye : but wherever light has reached the pale silver salts they have begun to turn to black silver . The film is kept in the dark , and treated with a chemical solution which develops and accelerates this change until a strong visible black silver image appears . Still in darkness , it is then treated with another kind of chemical - a fixer . This desensitizes the remaining white silver salts unaffected by light , allowing them to dissolve away by washing . You are left with a stable negative image , which can then be safely exposed to light without any further change occurring .
Turning the negative into a recognizable positive picture is quite simple . If you shine a beam of light through the negative . you can project the image on to another light - sensitive material -a sheet of photographic paper . This time , the dark areas of the negative will produce little response in the silver salts , while the light areas will produce a strong dark image . So after the paper has been developed and fixed you have a positive image a picture with light and dark parts of the original subject correctly reproduced . Mid - tones in the subject appear as a range of grays . because the light has only partially darkened the silver salts . This negative / positive process is very useful you can enlarge the image , alter its tones , and take as many copies from a single negative as you like .
One further problem remained . Silver salts are sensitive only to blue light . So they respond to blue and white light ( which contains blue light ) , but " see " red and green , the other two . primaries , as " no light " - that is , indistinguishable from black . Not until the end of the nineteenth century was it discovered that adding traces of dyes which absorb red and green could create a more normal , gray tone response to these colors . The result was panchromatic " film for black and white photography . The same discovery later led to color photography .
Pigments that absorb certain color wavelengths are also present in the human eye - in the cells sensitive to color which line the retina . In fact there are many features in common between how your eyes see , and how cameras and film work . But this does not mean your photographs will record the world just as your eyes see it and your brain interprets it . There are several basic differences - and these form the third basic element of photography , looked at on the next two pages .
Sensitivity to light :
Vegetation is light - sensitive Leaves use energy from sunlight to " photosynthesize " ( manufacture ) essential chemicals including the green pigment , chlorophyll . If you tape a black shape on to a living green leaf for several weeks it will leave a mark which is paler green .
Printing with sunlight When exposed to bright sunlight , some dyes and pigments begin to bleach , and cheap papers often discolor . You can see this by placing an object , such as a leaf , on a piece of newspaper in the sun . The cheap paper will become yellow except where protected from the action of the light by the object .
Sunburn Skin is sensitive to the sun's rays . Both visible and ultra - violet radiation stimulate it to produce a dark
pigment , visible as tanning . Shapes on the skin protected from the sun . such as a wristwatch , are clearly visible as lighter areas .
Exposing photographic paper to light Most salts of silver darken under the action of light . They break down into finely divided dark metallic silver . even without any processing . You can see this by taking a sheet of photographic printing paper from its packet in the dark , and positioning an opaque object such as a bunch of keys on its sensitive surface . Take this out into sunlight and let it stand .
Gradually the light - sensitive salts turn gray or purple ( a rich black is never reached ) . When you remove the keys , their shape remains visible on the paper as a white , negative image . But the white shape is not permanent and begins to darken once it is no longer hidden from light . Instead of using a long exposure to imprint the image direct , you can give a brief exposure under weaker light and then amplify the image to a rich black using developer chemicals . The result is a bold , permanent negative image .
Obtaining a positive from a negative image
A simple shape like a bunch of keys may be acceptable as a negative image , but pictures taken in the camera would look very odd if left with all their light and dark areas tonally reversed . To convert the negative into a positive , you simply print in on to another sheet of light sensitive material . Take the paper negative of the keys , top left , and press it face down on to photographic printing paper . Then direct a lamp through it for about a minute . After processing , the new sheet of paper will carry a " negative of a negative " - a positive image , left .
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