تويت
التصوير الفوتوغرافي الحديث
تحديد الوضع الصحيح ..
أفلام للتصوير الأسود والأبيض ..
المستحلب الفوتوغرافي ..
خصائص الفيلم ..
FILMS FOR BLACK - AND - WHITE PHOTOGRAPHY
Some of the material in this Assignment may be a little too advanced for you unless you have been using a camera for some time . If it is a trifle over your head , don't be afraid of it . Try to learn as much as you can from it now - you really need a lot of this information if you expect to do a good job of developing your negatives .
After you have learned as much as you can now , put this Assignment away for a time . Then read it again and you will learn still more from it .
NEW FROSTED 100W BULB
Fig . 21 ( Courtesy of W. T. Chester )
THE PHOTOGRAPHIC EMULSION
Before we jump right into the subject of film characteristics , let us first spend a few minutes in discussing the fundamental principles of the photographic emulsion - what it is and how it works . Such knowledge will help you immeasurably in your later Assignments , and will make it much easier for you to use good judgment in taking pictures and making prints from your negatives . More than that - it is intensely interesting in its own right .
You have all eaten gelatin desserts - and most of you know how such desserts are prepared . You simply take a little gelatin and dissolve it in hot water . Then you add a little sugar , some coloring and flavor , pour it into molds , and let it cool . When the gelatin is cold , it sets into a jelly - like mass with all the ingredients dissolved or suspended uniformly throughout .
The sensitive film you use in your camera is very much the same sort of thing . It consists of hard gelatin ( instead of the soft gelatins used for food ) in which are suspended some silver salts , mainly silver bromide ( instead of sugar and flavoring ) , and the entire mass is poured in a thin layer onto sheets of glass or plastic instead of into molds .
HOW THE EMULSION WORKS . So much for the physical make - up of the film you use in your camera . The next question is - how does it work ?
To answer that , we needn't become involved in a lot of difficult and complicated theory . If we put a small section of the film under the microscope , we find that it consists of a large number of very fine particles of cream - colored silver bromide scattered at random throughout the gelatin . As you would expect , these particles are not all the same size . Some are considerably larger than others . This difference in size is very important as we will see later .
Silver bromide , like many other silver salts , has an unusual characteristic . When light strikes it , it tends to break down and decompose into the elements of which it is composed - that is , the metal silver and the element bromine . We needn't be concerned with what happens to the bromine . It's probably absorbed by the remaining emulsion and reacts with it chemically . The important thing to us right now is that when light strikes the silver bromide of the film emulsion , a tiny portion of the silver bromide becomes metallic silver . It takes tremendous quantities of light to produce enough silver to be seen .
What happens upon ordinary exposure to light in a camera is something like this : The larger silver bromide particles are less stable - more delicately balanced -than the smaller ones . As a result , when light strikes the emulsion , they are the first to start breaking down . As exposure is increased ( as more light strikes the emulsion ) , some of the smaller particles begin to break down . After a certain value of exposure is reached , a majority of the particles have started to change .
All this is still on a very minute scale , so that it is impossible to see anything , but we can prove that these infinitesimally small particles of silver exist if we care to take the trouble . If our sensitive emulsion is spread on a sheet of plastic and put into a camera , and if the exposure to light is through a lens that projects an image onto the emulsion , the invisible collection of tiny particles of silver becomes a latent image . There are a number of ways to build up this latent image so that it becomes visible . We can put the film into various solutions which continue the action started by the light so that more and more of the silver bromide particles break down and produce a dense image of pure silver . This process is called development and will be covered in later Assignments .
FILM CHARACTERISTICS
We are now ready to get down to practical matters . The sensitive material we use in our camera is , of course , a far cry from the simple emulsion we have discussed . The actual preparation of any emulsion is a long , complicated , and exceedingly difficult operation . By such means as using small amounts of silver chloride and iodide as well as bromide , by washing the emulsion in various dyes . and other chemical solutions , by " ripening " with heat or ammonia , it is possible to produce emulsions that differ widely among themselves .
In this way , negative materials may be designed for various specific purposes . In order to study these differences so that you can always be sure of choosing the best film for any specific job , we will classify film characteristics under two heads : photographic and physical . Photographic characteristics cover such matters as color sensitivity , speed and graininess . Physical characteristics cover such matters as type of base , style of packaging , anti - halation features , and so on .
You must never lose sight of the fact that no single emulsion is best for all purposes . To design the fastest possible emulsion , it is generally necessary to sacrifice something else - such as exposure latitude , contrast , or fine grain ; to get exceptionally fine grain it may be necessary to sacrifice speed ; to get the greatest possible exposure latitude , it may be necessary to sacrifice speed and definition . Aside from such unavoidable compromises , you may want film with specific characteristics for a particular job . If you have to copy a line drawing , you want a film with a lot of contrast ; on the other hand if you make a portrait , you want a fairly soft working film - a film with too much contrast won't be suitable at all .
تحديد الوضع الصحيح ..
أفلام للتصوير الأسود والأبيض ..
المستحلب الفوتوغرافي ..
خصائص الفيلم ..
FILMS FOR BLACK - AND - WHITE PHOTOGRAPHY
Some of the material in this Assignment may be a little too advanced for you unless you have been using a camera for some time . If it is a trifle over your head , don't be afraid of it . Try to learn as much as you can from it now - you really need a lot of this information if you expect to do a good job of developing your negatives .
After you have learned as much as you can now , put this Assignment away for a time . Then read it again and you will learn still more from it .
NEW FROSTED 100W BULB
Fig . 21 ( Courtesy of W. T. Chester )
THE PHOTOGRAPHIC EMULSION
Before we jump right into the subject of film characteristics , let us first spend a few minutes in discussing the fundamental principles of the photographic emulsion - what it is and how it works . Such knowledge will help you immeasurably in your later Assignments , and will make it much easier for you to use good judgment in taking pictures and making prints from your negatives . More than that - it is intensely interesting in its own right .
You have all eaten gelatin desserts - and most of you know how such desserts are prepared . You simply take a little gelatin and dissolve it in hot water . Then you add a little sugar , some coloring and flavor , pour it into molds , and let it cool . When the gelatin is cold , it sets into a jelly - like mass with all the ingredients dissolved or suspended uniformly throughout .
The sensitive film you use in your camera is very much the same sort of thing . It consists of hard gelatin ( instead of the soft gelatins used for food ) in which are suspended some silver salts , mainly silver bromide ( instead of sugar and flavoring ) , and the entire mass is poured in a thin layer onto sheets of glass or plastic instead of into molds .
HOW THE EMULSION WORKS . So much for the physical make - up of the film you use in your camera . The next question is - how does it work ?
To answer that , we needn't become involved in a lot of difficult and complicated theory . If we put a small section of the film under the microscope , we find that it consists of a large number of very fine particles of cream - colored silver bromide scattered at random throughout the gelatin . As you would expect , these particles are not all the same size . Some are considerably larger than others . This difference in size is very important as we will see later .
Silver bromide , like many other silver salts , has an unusual characteristic . When light strikes it , it tends to break down and decompose into the elements of which it is composed - that is , the metal silver and the element bromine . We needn't be concerned with what happens to the bromine . It's probably absorbed by the remaining emulsion and reacts with it chemically . The important thing to us right now is that when light strikes the silver bromide of the film emulsion , a tiny portion of the silver bromide becomes metallic silver . It takes tremendous quantities of light to produce enough silver to be seen .
What happens upon ordinary exposure to light in a camera is something like this : The larger silver bromide particles are less stable - more delicately balanced -than the smaller ones . As a result , when light strikes the emulsion , they are the first to start breaking down . As exposure is increased ( as more light strikes the emulsion ) , some of the smaller particles begin to break down . After a certain value of exposure is reached , a majority of the particles have started to change .
All this is still on a very minute scale , so that it is impossible to see anything , but we can prove that these infinitesimally small particles of silver exist if we care to take the trouble . If our sensitive emulsion is spread on a sheet of plastic and put into a camera , and if the exposure to light is through a lens that projects an image onto the emulsion , the invisible collection of tiny particles of silver becomes a latent image . There are a number of ways to build up this latent image so that it becomes visible . We can put the film into various solutions which continue the action started by the light so that more and more of the silver bromide particles break down and produce a dense image of pure silver . This process is called development and will be covered in later Assignments .
FILM CHARACTERISTICS
We are now ready to get down to practical matters . The sensitive material we use in our camera is , of course , a far cry from the simple emulsion we have discussed . The actual preparation of any emulsion is a long , complicated , and exceedingly difficult operation . By such means as using small amounts of silver chloride and iodide as well as bromide , by washing the emulsion in various dyes . and other chemical solutions , by " ripening " with heat or ammonia , it is possible to produce emulsions that differ widely among themselves .
In this way , negative materials may be designed for various specific purposes . In order to study these differences so that you can always be sure of choosing the best film for any specific job , we will classify film characteristics under two heads : photographic and physical . Photographic characteristics cover such matters as color sensitivity , speed and graininess . Physical characteristics cover such matters as type of base , style of packaging , anti - halation features , and so on .
You must never lose sight of the fact that no single emulsion is best for all purposes . To design the fastest possible emulsion , it is generally necessary to sacrifice something else - such as exposure latitude , contrast , or fine grain ; to get exceptionally fine grain it may be necessary to sacrifice speed ; to get the greatest possible exposure latitude , it may be necessary to sacrifice speed and definition . Aside from such unavoidable compromises , you may want film with specific characteristics for a particular job . If you have to copy a line drawing , you want a film with a lot of contrast ; on the other hand if you make a portrait , you want a fairly soft working film - a film with too much contrast won't be suitable at all .
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