تويت
التصوير الفوتوغرافي الحديث
تحديد الوضع الصحيح ..
خصائص التصوير الفوتوغرافي ..
PHOTOGRAPHIC PROPERTIES
SPEED . There is a very wide variation in the sensitivity ( speed ) of various photographic emulsions , and the wise photographer selects the film speed best suited for the job at hand . For example , if you have to take pictures at a football game and you know the sky will be dull and the light bad , you will load your camera with a high speed film . On the other hand , if you are going to take some landscapes in bright sunlight or some portraits in the studio where you have lots of light , you will use a medium speed film which may not be quite so fast but which has greater latitude and better over - all tonal gradation . If you want to copy a picture in the studio where you can expose as long as necessary , and you want to develop the negative by inspection to give it individual attention , you can load the camera with a slow film so that you can use a brighter safelight in the darkroom .
At first , there was very little accurate information available about the speed of photographic emulsions . They were simply called slow , fast , extra rapid , ultra - speed , and so on . In a short time , however , this system became entirely too clumsy and inaccurate . Each time a faster film appeared on the market , it became a problem to select a name which meant more speed . At last , it was evident that only an accurate system of speed rating would give the photographer definite data about film speed .
The first of these speed rating systems was the H & D system , named after two men , Hurter and Driffield , who were among the first to devise an acceptable means of measuring the speed of photographic emulsions . They subjected a strip of film to a series of measured exposures , each one a definite increase over the previous one , developed the film under standard conditions with a standard developer , and then measured the density produced by each exposure . They then plotted a curve from their information and used this curve to indicate film speed . Figure 22 shows such a curve .
The H & D curve ( also called the characteristic curve ) gave a great deal of extremely useful information about the emulsion , and even today this curve is the most important single item of information about a photographic emulsion . However , the system of film speeds used by Hurter and Driffield had a number of drawbacks , and it was eventually dropped by film manufacturers . There are now a number of systems , each claiming its own set of advantages . All have one thing in common - they start with the film characteristic curve , but they differ somewhat in how they interpret the curve or what part of the curve they use as a basis for their numbering system . We can leave the fine points of difference to the technician - we will use the ASA system , since it is standard in the United States .
COLOR SENSITIVITY . The normal eye is sensitive to all colors of the spectrum from red to violet , but this is not true of all photographic emulsions . As a matter of fact , plain silver bromide is sensitive only to blue and violet . It has been found that if certain sensitizing dyes are added , an emulsion becomes sensitive to other colors , such as green , orange , yellow , and red . An emulsion which is made sensitive to green , yellow , and orange by the use of such dyes is called an orthochromatic emulsion ; if it is also made sensitive to red , it is called a panchromatic emulsion . All common film today is panchromatic .
Fig . 22 H & D curve for a black and white film . A in dicates the scene brightness range , and B shows the location of a thin , average , and dense negative on the curve .
It is possible to sensitize an emulsion so it is sensitive even to the invisible infrared rays . Infrared is not light in the true sense , since we cannot see it , but it is often referred to as light because it comes right next to red light in the spectrum . Because it is invisible to the eye , it is often called black light , but actually it is really radiant heat - the same sort of thing you feel when you put your hands in front of a hot fire to warm them . Infrared film , although it is sensitive to infrared light , is still highly sensitive to ordinary light , so if you use it normally in the camera , you will get almost the same results as from any slow panchromatic film . However , if you use an infrared filter in front of the lens so as to filter out all the visible light , you will get the unusual results associated with infrared photography . This subject is discussed again in Assignment 10 when we study color filters .
Panchromatic film , although sensitive to all colors , is still more sensitive to blue than to red . If you have an older exposure meter , or if you are using older film - speed tables , you will note that two speed ratings are given for each film - one rating for daylight , and a lower rating for tungsten light . This is because daylight is bluer than tungsten light , and film is more sensitive to blue . However , since the present panchromatic films have more nearly the same response to all colors , and the meters themselves now have about the same color response as the eye , present meters will give the correct reading whether the film is used indoors or out . Newer film - speed tables now show the same speed rating for daylight as for tungsten light . This , of course , is true only for panchromatic film - orthochromatic film still has a lower speed rating for tungsten light than for daylight .
EMULSION CONTRAST . Certain emulsions are inherently more contrasty than others . The low contrast emulsions are generally used for portraiture , the medium contrast for all around amateur and commercial work , and the high contrast for copying line drawings and similar work where the negatives must be pure black - and - white , with no half - tones whatever .
FINE - GRAIN FILMS . Fine - grain films are particularly important to the 35mm photographer because he must enlarge every negative to a considerable degree . Fine - grain film is not important to the user of a 4 x 5 camera . It means little , too , to the man who uses a 24 x 24 camera , unless he expects to enlarge small sections of his negatives beyond 8 x 10. As a matter of fact , fine - grain film is difficult to obtain in anything but the 35mm size , except in supply houses catering to the needs of the commercial photographer .
In addition to being inherently finer grained , modern fine grain film has a much thinner emulsion than is used in a more conventional film . Figures 23 and 24 illustrate why this also helps produce a sharper image . In Figure 23 , a lens is shown projecting an image of point A onto the surface of a conventional medium - grain film . The normal emulsion layer is shown greatly exaggerated . Since the emulsion is not opaque , the light does not stop on the surface of the film , but penetrates all the way through . All the silver in the cone of light within the emulsion layer is exposed . When the film is déveloped , the image of point A is not a point on the surface of the film , but a circle with the diameter ( a ) . In Figure 24 , we have exactly the same situation , only in a film with a much thinner emulsion . Now the circle has the smaller diameter , ( b ) . In other wor ecause there is less spread of light or diffusion within the emulsion , the picture will be sharper .
Fig . 23 Because light penetrates into emulsion , a point on surface of emulsion results in a tiny circle .
The individual crystals of silver bromide in a fine - grain emulsion are smaller than the crystals in a medium - grain film . These crystals , however , are not the grain you see in a developed negative or print . The grain in the finished photograph is caused by the accumulation of clusters or clumps of silver during development . But the smaller grain size of the silver bromide particles does contribute to finer grain in the developed negative .
Fig . 24 In thin - emulsion films circle is smaller , thus producing a sharper image .
The smaller crystals in fine - grain film are more stable , and are less easily broken down by the action of light . As a result , fine - grain film is slower than medium - grain or high - speed film . In addition , because fine - grain film has a thinner emulsion ( less silver bromide ) , it has less exposure latitude . Three times overexposure will hardly be noticeable in a general purpose film . In a fine - grain , thin - emulsion film , however , the same degree of overexposure may produce blocked - up highlights .
To sum it up , thin - emulsion , fine - grain films will produce sharper prints under extreme enlargement . They do have the disadvantage of slower speed and less exposure latitude . In Figures 25 and 26 we see the type of subjects which can be photographed to advantage on thin - emulsion film where the negative is to be greatly enlarged .
Fig . 25 Closely spaced , fine lines of rigging will appear at their sharpest when photographed with a thin emulsion , fine - grain film .
EXPOSURE LATITUDE , One reason why it is best to avoid the use of the fastest films whenever possible is that greater latitude in exposure is found in emulsions other than the extremely fast ones . For general work outdoors and in a well - lighted commercial studio , there is seldom any need to use ultra - speed film . You should do very well with the standard . general - purpose emulsions .
For amateur use , medium speed roll film is usually double - coated , first with a slow emulsion , and then with a faster emulsion on top of the slow one . The fast emulsion gives the film its speed and picks up detail in the shadows , while the bottom layer of slow emulsion prevents overexposure of the bright highlights . The latitude of such films is so great that they may be greatly over exposed and still give printable pictures . However , even though it is possible to overexpose such films , it isn't a good idea to do so . The negatives aren't quite so good , for one thing , and the graininess is increased for another .
DEVELOPMENT SPEED . All films do not develop at the same rate . Some emulsions are basically slow to develop and so must be kept longer in the developing solution . Generally , these are the very fast , less contrasty emulsions . For that reason , it is important that you follow the manufacturer's instructions and develop each film for the recommended time and at the recommended temperature . Film packs generally require 10 % longer development than roll film using the same emulsion because the film pack has an extra top coating of tough gelatin to protect the emulsion when the film is pulled around to the back of the pack after exposure .
Fig . 26 Fine detail in background buildings can be best reproduced with a thin - emulsion , fine - grain film if large prints are to be made .
HALATION . The emulsion of a negative film is semi - transparent so that if it is exposed to a very strong light , some of the light may go all the way through to the base . If this light is reflected back into the emulsion from the base of the film , it will produce a veil of light somewhat resembling a halo around a bright object . This is called halation . To prevent halation , a black or deep green dye is mixed with the gelatin layer with which the back of the film is coated to prevent it from curling . The dye absorbs all the actinic light that may penetrate the emulsion ; thus the back coating prevents halation . The dye bleaches out in the developer and fixing baths .
In 35mm film , which has no gelatin backing , a blue - gray dye is applied to the film base itself . This dye does not wash out , but is so light in color that it does not affect printing quality .
تحديد الوضع الصحيح ..
خصائص التصوير الفوتوغرافي ..
PHOTOGRAPHIC PROPERTIES
SPEED . There is a very wide variation in the sensitivity ( speed ) of various photographic emulsions , and the wise photographer selects the film speed best suited for the job at hand . For example , if you have to take pictures at a football game and you know the sky will be dull and the light bad , you will load your camera with a high speed film . On the other hand , if you are going to take some landscapes in bright sunlight or some portraits in the studio where you have lots of light , you will use a medium speed film which may not be quite so fast but which has greater latitude and better over - all tonal gradation . If you want to copy a picture in the studio where you can expose as long as necessary , and you want to develop the negative by inspection to give it individual attention , you can load the camera with a slow film so that you can use a brighter safelight in the darkroom .
At first , there was very little accurate information available about the speed of photographic emulsions . They were simply called slow , fast , extra rapid , ultra - speed , and so on . In a short time , however , this system became entirely too clumsy and inaccurate . Each time a faster film appeared on the market , it became a problem to select a name which meant more speed . At last , it was evident that only an accurate system of speed rating would give the photographer definite data about film speed .
The first of these speed rating systems was the H & D system , named after two men , Hurter and Driffield , who were among the first to devise an acceptable means of measuring the speed of photographic emulsions . They subjected a strip of film to a series of measured exposures , each one a definite increase over the previous one , developed the film under standard conditions with a standard developer , and then measured the density produced by each exposure . They then plotted a curve from their information and used this curve to indicate film speed . Figure 22 shows such a curve .
The H & D curve ( also called the characteristic curve ) gave a great deal of extremely useful information about the emulsion , and even today this curve is the most important single item of information about a photographic emulsion . However , the system of film speeds used by Hurter and Driffield had a number of drawbacks , and it was eventually dropped by film manufacturers . There are now a number of systems , each claiming its own set of advantages . All have one thing in common - they start with the film characteristic curve , but they differ somewhat in how they interpret the curve or what part of the curve they use as a basis for their numbering system . We can leave the fine points of difference to the technician - we will use the ASA system , since it is standard in the United States .
COLOR SENSITIVITY . The normal eye is sensitive to all colors of the spectrum from red to violet , but this is not true of all photographic emulsions . As a matter of fact , plain silver bromide is sensitive only to blue and violet . It has been found that if certain sensitizing dyes are added , an emulsion becomes sensitive to other colors , such as green , orange , yellow , and red . An emulsion which is made sensitive to green , yellow , and orange by the use of such dyes is called an orthochromatic emulsion ; if it is also made sensitive to red , it is called a panchromatic emulsion . All common film today is panchromatic .
Fig . 22 H & D curve for a black and white film . A in dicates the scene brightness range , and B shows the location of a thin , average , and dense negative on the curve .
It is possible to sensitize an emulsion so it is sensitive even to the invisible infrared rays . Infrared is not light in the true sense , since we cannot see it , but it is often referred to as light because it comes right next to red light in the spectrum . Because it is invisible to the eye , it is often called black light , but actually it is really radiant heat - the same sort of thing you feel when you put your hands in front of a hot fire to warm them . Infrared film , although it is sensitive to infrared light , is still highly sensitive to ordinary light , so if you use it normally in the camera , you will get almost the same results as from any slow panchromatic film . However , if you use an infrared filter in front of the lens so as to filter out all the visible light , you will get the unusual results associated with infrared photography . This subject is discussed again in Assignment 10 when we study color filters .
Panchromatic film , although sensitive to all colors , is still more sensitive to blue than to red . If you have an older exposure meter , or if you are using older film - speed tables , you will note that two speed ratings are given for each film - one rating for daylight , and a lower rating for tungsten light . This is because daylight is bluer than tungsten light , and film is more sensitive to blue . However , since the present panchromatic films have more nearly the same response to all colors , and the meters themselves now have about the same color response as the eye , present meters will give the correct reading whether the film is used indoors or out . Newer film - speed tables now show the same speed rating for daylight as for tungsten light . This , of course , is true only for panchromatic film - orthochromatic film still has a lower speed rating for tungsten light than for daylight .
EMULSION CONTRAST . Certain emulsions are inherently more contrasty than others . The low contrast emulsions are generally used for portraiture , the medium contrast for all around amateur and commercial work , and the high contrast for copying line drawings and similar work where the negatives must be pure black - and - white , with no half - tones whatever .
FINE - GRAIN FILMS . Fine - grain films are particularly important to the 35mm photographer because he must enlarge every negative to a considerable degree . Fine - grain film is not important to the user of a 4 x 5 camera . It means little , too , to the man who uses a 24 x 24 camera , unless he expects to enlarge small sections of his negatives beyond 8 x 10. As a matter of fact , fine - grain film is difficult to obtain in anything but the 35mm size , except in supply houses catering to the needs of the commercial photographer .
In addition to being inherently finer grained , modern fine grain film has a much thinner emulsion than is used in a more conventional film . Figures 23 and 24 illustrate why this also helps produce a sharper image . In Figure 23 , a lens is shown projecting an image of point A onto the surface of a conventional medium - grain film . The normal emulsion layer is shown greatly exaggerated . Since the emulsion is not opaque , the light does not stop on the surface of the film , but penetrates all the way through . All the silver in the cone of light within the emulsion layer is exposed . When the film is déveloped , the image of point A is not a point on the surface of the film , but a circle with the diameter ( a ) . In Figure 24 , we have exactly the same situation , only in a film with a much thinner emulsion . Now the circle has the smaller diameter , ( b ) . In other wor ecause there is less spread of light or diffusion within the emulsion , the picture will be sharper .
Fig . 23 Because light penetrates into emulsion , a point on surface of emulsion results in a tiny circle .
The individual crystals of silver bromide in a fine - grain emulsion are smaller than the crystals in a medium - grain film . These crystals , however , are not the grain you see in a developed negative or print . The grain in the finished photograph is caused by the accumulation of clusters or clumps of silver during development . But the smaller grain size of the silver bromide particles does contribute to finer grain in the developed negative .
Fig . 24 In thin - emulsion films circle is smaller , thus producing a sharper image .
The smaller crystals in fine - grain film are more stable , and are less easily broken down by the action of light . As a result , fine - grain film is slower than medium - grain or high - speed film . In addition , because fine - grain film has a thinner emulsion ( less silver bromide ) , it has less exposure latitude . Three times overexposure will hardly be noticeable in a general purpose film . In a fine - grain , thin - emulsion film , however , the same degree of overexposure may produce blocked - up highlights .
To sum it up , thin - emulsion , fine - grain films will produce sharper prints under extreme enlargement . They do have the disadvantage of slower speed and less exposure latitude . In Figures 25 and 26 we see the type of subjects which can be photographed to advantage on thin - emulsion film where the negative is to be greatly enlarged .
Fig . 25 Closely spaced , fine lines of rigging will appear at their sharpest when photographed with a thin emulsion , fine - grain film .
EXPOSURE LATITUDE , One reason why it is best to avoid the use of the fastest films whenever possible is that greater latitude in exposure is found in emulsions other than the extremely fast ones . For general work outdoors and in a well - lighted commercial studio , there is seldom any need to use ultra - speed film . You should do very well with the standard . general - purpose emulsions .
For amateur use , medium speed roll film is usually double - coated , first with a slow emulsion , and then with a faster emulsion on top of the slow one . The fast emulsion gives the film its speed and picks up detail in the shadows , while the bottom layer of slow emulsion prevents overexposure of the bright highlights . The latitude of such films is so great that they may be greatly over exposed and still give printable pictures . However , even though it is possible to overexpose such films , it isn't a good idea to do so . The negatives aren't quite so good , for one thing , and the graininess is increased for another .
DEVELOPMENT SPEED . All films do not develop at the same rate . Some emulsions are basically slow to develop and so must be kept longer in the developing solution . Generally , these are the very fast , less contrasty emulsions . For that reason , it is important that you follow the manufacturer's instructions and develop each film for the recommended time and at the recommended temperature . Film packs generally require 10 % longer development than roll film using the same emulsion because the film pack has an extra top coating of tough gelatin to protect the emulsion when the film is pulled around to the back of the pack after exposure .
Fig . 26 Fine detail in background buildings can be best reproduced with a thin - emulsion , fine - grain film if large prints are to be made .
HALATION . The emulsion of a negative film is semi - transparent so that if it is exposed to a very strong light , some of the light may go all the way through to the base . If this light is reflected back into the emulsion from the base of the film , it will produce a veil of light somewhat resembling a halo around a bright object . This is called halation . To prevent halation , a black or deep green dye is mixed with the gelatin layer with which the back of the film is coated to prevent it from curling . The dye absorbs all the actinic light that may penetrate the emulsion ; thus the back coating prevents halation . The dye bleaches out in the developer and fixing baths .
In 35mm film , which has no gelatin backing , a blue - gray dye is applied to the film base itself . This dye does not wash out , but is so light in color that it does not affect printing quality .
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