تويت
التصوير الفوتوغرافي الحديث
تطوير السلبية ..
النقاط الدقيقة في معالجة الفيلم ..
حمام التوقف ..
حمام التثبيت ..
تحديد طول وقت التثبيت ..
غسل الفيلم ..
تجفيف الفيلم ..
THE STOP BATH
The stop bath is used between developing and fixing . It may be plain water or a mild acid solution . It has two important functions : it stops the developing action , and it rinses out most of the developer and thus reduces contamination of the fixing solution .
For most film developers , which are quite slow compared to paper developers , the stopping action of the acid is secondary , and a plain water rinse is almost always best . A strong acid stop bath must not be used for film developers if they contain a high concentration of sodium carbonate or sodium sulfite , and most film developers do . If a gelatin emulsion containing such a developer is immersed in a strongly acid stop bath , bubbles of carbon dioxide or sulfur dioxide gas form . These gas bubbles disrupt the gelatin and increase grain size . In extreme cases , this action may cause small pinholes in the emulsion .
If the film developer is quick - acting , a mild acid stop bath may be used . Proper strength is one part of 28 % acetic acid to 50 parts of water ( 1/3 ounce acid to 16 ounces water ) . This is less than the strength of acid which may safely be used for photographic paper .
THE FIXING BATH
After development , the film still contains all the unexposed silver bromide . Unless this silver bromide is removed , it will gradually turn black on further exposure to light . To make the picture permanent , you must fix it by dissolving away this silver bromide . A standard fixing formula will contain a number of ingredients with which the photographer should be acquainted .
THE FIXING AGENT . This is the chemical which actually dissolves the undeveloped silver bromide . The most common fixing agent is sodium thiosulfate , commonly called hypo in the photographic industry . Another popular fixing agent is ammonium thiosulfate which is much more rapid in its action and is the base of most quick - fixing solutions .
ACID . When film is transferred to the fixer , it carries with it some developer even when a rinse bath is used . This developer will gradually oxidize and turn dark . When a fixer has been contaminated with oxidized developer , it stains and is unfit for use . However , if the fixing solution contains an acid , it neutralizes the alkali of the developer and prevents it from turning dark . Almost any common acid will do . Acetic acid is used most frequently in liquid fixers , and citric acid is most commonly used in packaged , powdered fixers .
SODIUM SULFITE . Unfortunately , the two best fixing agents , hypo and ammonium thiosulfate , would both be destroyed by acid if they weren't given some special protection . This protection is provided by sodium sulfite . If sodium sulfite is first added to the solution , the acid does not attack the fixing agent . All common fixing solutions , therefore , contain sodium sulfite or its equivalent .
HARDENER . The gelatin emulsion of a newly developed film is quite soft and easily damaged . It is general practice , therefore , to add a hardening agent such as potassium alum to the fixer so the emulsion hardens while it is being fixed .
ANTI - SLUDGING AGENT . Potassium alum separates out of a fixing solution as a sludge when too much alkali is carried over into the hypo . The useful life of a fixer can , therefore , be increased if an anti - sludging agent is added . Boric acid is most commonly used for this purpose .
DETERMINING LENGTH OF FIXING TIME
Correct fixing time is important . Underfixing causes negatives and prints to stain and discolor in time . Overfixing dissolves some of the silver image , weakening the lightly exposed areas where detail is most important . Overfixing also greatly increases the time required for effective washing of prints . This will be discussed later , under print making .
Fixing proceeds in two stages . In the first half of the fixing period , the silver bromide in the emulsion is converted into intermediate silver compounds which are colorless , but which dissolve slowly . In the second half of the fixing period , these silver compounds are converted to others which dissolve much more readily . and can be easily washed out . For that reason , it is necessary to fix films for twice the time it takes them to clear .
A fresh hypo fixer should clear a film in about five minutes , and fix it in ten . As the bath becomes exhausted , fixing action slows up . When clearing time : approaches 8 minutes , which means 16 minutes for complete fixing , the bath should be thrown away . Rapid fixers , as distinct from ordinary hypo fixers , fix in only two or three minutes when used full strength . Generally it is safer to allow four minutes . The solution should be discarded when it takes 4 minutes to clear ( a total of 8 minutes to fix ) .
WASHING THE FILM
After fixing , the gelatin emulsion is still heavily loaded with various chemicals which must be washed out completely if the image is to be permanent . Washing the film for 20 minutes in rapidly circulating water at a temperature of 60 ° to 70 ° F. is usually sufficient . However , if the wash water is below 60 ° º , or if the film is on a very compact reel and the water circulation around it is relatively poor , increase the time to 30 minutes .
DRYING THE FILM
Always dry your film in a dust - free room or cabinet . A dusty basement , particularly in winter near the furnace , is the worst possible place to dry film . Unfortunately , it is used all too frequently by students and amateurs with basement darkrooms .
Successful drying requires the removal of water at a uniform rate until the amount of moisture remaining in the film corresponds to the relative humidity of the air . It is not good practice to dry film with a strong blast of hot air . This not only increases negative density and contrast , but may produce a buckled , brittle film . The shortest time for safe drying in summer , when solutions are warm and the gelatin emulsion is badly swollen , is 30 minutes . This time may be somewhat shortened in winter , particularly with faster drying 35mm film . If you want to speed up drying with safety , increase the air movement past the film by using a fan , provided it doesn't raise dust . You can get still faster drying by using heat , but be sure the temperature of the air moving past the moist film surface is not over 100 ° F . Such rapid drying is not recommended if fine - grain results are wanted .
Once a film has started to dry , it should not be disturbed , since the rate of drying affects negative density . If a film has partially dried at a slow rate , and you place it in a current of warm air to speed things up , part of the film will be denser , and you may get a noticeable line of suddenly changing density between the two areas .
WETTING AGENTS . Wetting agents are used extensively by photographers in the final rinse before films are hung to dry . Water containing a wetting agent will not collect in droplets , but will spread out in a thin , uniform layer . If you soak film in a weak solution of a wetting agent after washing is completed , the surface water will run off when you hang the film to dry , and there will be no large water drops to leave drying marks . If you use a wetting agent , be sure to obtain it from your photographic dealer . There are wetting agents for other uses which leave an oily residue .
تطوير السلبية ..
النقاط الدقيقة في معالجة الفيلم ..
حمام التوقف ..
حمام التثبيت ..
تحديد طول وقت التثبيت ..
غسل الفيلم ..
تجفيف الفيلم ..
THE STOP BATH
The stop bath is used between developing and fixing . It may be plain water or a mild acid solution . It has two important functions : it stops the developing action , and it rinses out most of the developer and thus reduces contamination of the fixing solution .
For most film developers , which are quite slow compared to paper developers , the stopping action of the acid is secondary , and a plain water rinse is almost always best . A strong acid stop bath must not be used for film developers if they contain a high concentration of sodium carbonate or sodium sulfite , and most film developers do . If a gelatin emulsion containing such a developer is immersed in a strongly acid stop bath , bubbles of carbon dioxide or sulfur dioxide gas form . These gas bubbles disrupt the gelatin and increase grain size . In extreme cases , this action may cause small pinholes in the emulsion .
If the film developer is quick - acting , a mild acid stop bath may be used . Proper strength is one part of 28 % acetic acid to 50 parts of water ( 1/3 ounce acid to 16 ounces water ) . This is less than the strength of acid which may safely be used for photographic paper .
THE FIXING BATH
After development , the film still contains all the unexposed silver bromide . Unless this silver bromide is removed , it will gradually turn black on further exposure to light . To make the picture permanent , you must fix it by dissolving away this silver bromide . A standard fixing formula will contain a number of ingredients with which the photographer should be acquainted .
THE FIXING AGENT . This is the chemical which actually dissolves the undeveloped silver bromide . The most common fixing agent is sodium thiosulfate , commonly called hypo in the photographic industry . Another popular fixing agent is ammonium thiosulfate which is much more rapid in its action and is the base of most quick - fixing solutions .
ACID . When film is transferred to the fixer , it carries with it some developer even when a rinse bath is used . This developer will gradually oxidize and turn dark . When a fixer has been contaminated with oxidized developer , it stains and is unfit for use . However , if the fixing solution contains an acid , it neutralizes the alkali of the developer and prevents it from turning dark . Almost any common acid will do . Acetic acid is used most frequently in liquid fixers , and citric acid is most commonly used in packaged , powdered fixers .
SODIUM SULFITE . Unfortunately , the two best fixing agents , hypo and ammonium thiosulfate , would both be destroyed by acid if they weren't given some special protection . This protection is provided by sodium sulfite . If sodium sulfite is first added to the solution , the acid does not attack the fixing agent . All common fixing solutions , therefore , contain sodium sulfite or its equivalent .
HARDENER . The gelatin emulsion of a newly developed film is quite soft and easily damaged . It is general practice , therefore , to add a hardening agent such as potassium alum to the fixer so the emulsion hardens while it is being fixed .
ANTI - SLUDGING AGENT . Potassium alum separates out of a fixing solution as a sludge when too much alkali is carried over into the hypo . The useful life of a fixer can , therefore , be increased if an anti - sludging agent is added . Boric acid is most commonly used for this purpose .
DETERMINING LENGTH OF FIXING TIME
Correct fixing time is important . Underfixing causes negatives and prints to stain and discolor in time . Overfixing dissolves some of the silver image , weakening the lightly exposed areas where detail is most important . Overfixing also greatly increases the time required for effective washing of prints . This will be discussed later , under print making .
Fixing proceeds in two stages . In the first half of the fixing period , the silver bromide in the emulsion is converted into intermediate silver compounds which are colorless , but which dissolve slowly . In the second half of the fixing period , these silver compounds are converted to others which dissolve much more readily . and can be easily washed out . For that reason , it is necessary to fix films for twice the time it takes them to clear .
A fresh hypo fixer should clear a film in about five minutes , and fix it in ten . As the bath becomes exhausted , fixing action slows up . When clearing time : approaches 8 minutes , which means 16 minutes for complete fixing , the bath should be thrown away . Rapid fixers , as distinct from ordinary hypo fixers , fix in only two or three minutes when used full strength . Generally it is safer to allow four minutes . The solution should be discarded when it takes 4 minutes to clear ( a total of 8 minutes to fix ) .
WASHING THE FILM
After fixing , the gelatin emulsion is still heavily loaded with various chemicals which must be washed out completely if the image is to be permanent . Washing the film for 20 minutes in rapidly circulating water at a temperature of 60 ° to 70 ° F. is usually sufficient . However , if the wash water is below 60 ° º , or if the film is on a very compact reel and the water circulation around it is relatively poor , increase the time to 30 minutes .
DRYING THE FILM
Always dry your film in a dust - free room or cabinet . A dusty basement , particularly in winter near the furnace , is the worst possible place to dry film . Unfortunately , it is used all too frequently by students and amateurs with basement darkrooms .
Successful drying requires the removal of water at a uniform rate until the amount of moisture remaining in the film corresponds to the relative humidity of the air . It is not good practice to dry film with a strong blast of hot air . This not only increases negative density and contrast , but may produce a buckled , brittle film . The shortest time for safe drying in summer , when solutions are warm and the gelatin emulsion is badly swollen , is 30 minutes . This time may be somewhat shortened in winter , particularly with faster drying 35mm film . If you want to speed up drying with safety , increase the air movement past the film by using a fan , provided it doesn't raise dust . You can get still faster drying by using heat , but be sure the temperature of the air moving past the moist film surface is not over 100 ° F . Such rapid drying is not recommended if fine - grain results are wanted .
Once a film has started to dry , it should not be disturbed , since the rate of drying affects negative density . If a film has partially dried at a slow rate , and you place it in a current of warm air to speed things up , part of the film will be denser , and you may get a noticeable line of suddenly changing density between the two areas .
WETTING AGENTS . Wetting agents are used extensively by photographers in the final rinse before films are hung to dry . Water containing a wetting agent will not collect in droplets , but will spread out in a thin , uniform layer . If you soak film in a weak solution of a wetting agent after washing is completed , the surface water will run off when you hang the film to dry , and there will be no large water drops to leave drying marks . If you use a wetting agent , be sure to obtain it from your photographic dealer . There are wetting agents for other uses which leave an oily residue .
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