تويت
التصوير الفوتوغرافي الحديث
التصوير بالألوان وحقائق حول المرشحات
الجزء الثاني حقائق حول المرشحات
أنواع المرشحات
TYPES OF FILTERS
Color filters for black - and - white photography can serve a number of different purposes , and it is well for the student photographer to bear them clearly in mind . Filters for these purposes may be classified as follows :
1. Correction filters
2. Contrast filters
3. Haze filters
4. Neutral density filters
CORRECTION FILTERS . While panchromatic emulsions respond to all the colors the eye can see , they do not respond to these colors with quite the same relative values as does the eye . For example , blue normally appears darker to the
eye than green , yet a film is very sensitive to blue , and will record it as lighter than green .
By the use of suitable filters , the response of a film can be changed to reproduce colors in the same relative brightness as the eye sees them . For example , to overcome the excess sensitivity of a film to blue , it is necessary to use a filter which absorbs some of the blue . Such a filter appears light yellow . The very popular K2 is such a correction filter for regular panchromatic film . Some of the very fast panchromatic emulsions are excessively sensitive to red as well as to blue . To compensate for such excess red sensitivity , it is necessary to use a filter which absorbs not only some blue but also some red . Such a filter will appear light green . A suitable filter is the Wratten X1 .
In dealing with correction filters , the terms overcorrected and undercorrected are often used . Undercorrected means either that no filter was used , or that the filter was too light in color to fully correct the film to match the color response of the average eye . In an undercorrected landscape photograph , for example , the blue sky is too light , the green grass too dark . Overcorrection is the other extreme , and indicates that a strong filter was used to reverse the normal sensitivity of the film to the color in question . An overcorrected picture would have a very dark sky , as in Figure 11 .
Figure 11 Red filter was used to produce this overcorrected black sky . Photographed by S. J. Silverstein
CONTRAST FILTERS . Contrast filters are used to lighten or darken certain colors in the subject for the purpose of introducing a brightness difference between two colors . This means that one of the colors is made to photograph much lighter or darker than the other , thus increasing their relative contrast .
To the eye , objects are distinguished from their surroundings by two kinds of contrast - tone contrast and color contrast . A black - and - white photograph has only one kind of contrast - tone contrast . It is sometimes necessary to translate color contrast into tone contrast when taking a picture of a colored object . To achieve a specific purpose , the tone contrast may have to be subdued or exaggerated as desired . For example , a red apple and its green leaves may photograph as two nearly identical tones of gray . The color contrast - green against red - disappears in a black - and - white photograph , and only the tone contrast remains . The tone contrast in this case in quite low .
This is clearly shown in Figure 12 , which is a photograph of two plastic tumblers . The upright tumbler is green - the horizontal tumbler is red . The colors were carefully chosen so they have the same visual brightness . As a result the two tumblers photograph to the same shade of gray .
Figu 12 Green upright tumbler and red horizontal tum bler appear same tone of gray because both colors have same visual brightness .
To achieve a tone contrast that more nearly approaches the color contrast as it appears to the eye , one of the two tumblers should be made lighter than the other . If we use a red filter ( Figure 13 ) , the red ( horizontal ) tumbler will appear lighter , and the upright tumbler darker . This is because the red filter lets the red light reflected from the horizontal tumbler through with little interference , but absorbs a high proportion of the green light . Thus the original visual contrast between the two tumblers is restored . The same result , insofar as contrast is concerned , would be achieved by using a green filter ( Figure 14 ) . With this filter the green upright tumbler would become light and the red horizontal tumbler dark . Which of the two filters should be used is a question for the photographer to answer on the basis of individual circumstances .
The student who wishes to understand the use of contrast filters should never forget the following three basic principles :
1. A colored object photographs light if the film is exposed through a filter of the same color as the object itself . For example , consider photographing a Figure 13 Same as Figure 12 , but photographed through red filter Horizontal ( red ) tumbler appears lighter .
Figure 14 Same as Figure 12 , but photographed through green filter . Now . vertical ( green tumbler . appears lighter .
yellowed document . If you used a G ( deep yellow ) filter , which transmits yellow freely , the paper would be reproduced as white .
2. A colored object photographs dark if the negative is exposed through a filter which absorbs the color of the object . Such a filter allows only a little of the light reflected from the object to reach the film so it photographs quite dark . This principle can be used to advantage in photographing a document written in light blue ink . The blue should be made as dark as possible . Because a red filter absorbs blue light , the best contrast is obtained with a red filter . A deep yellow filter , such as the G , also gives good results except for very light blue ink . Also , its filter factor is less . For this reason you may prefer to use a G filter .
3. To show the best detail within a colored object , photograph it so it appears light . Thus dark green leaves show most detail when photographed
through a green filter . The grain pattern of a beautiful piece of red mahogany shows up best if photographed through a red filter . Table IV , page 38 , shows the transmission and absorption characteristics of various color filters .
HAZE FILTERS . Distant landscapes and aerial views from high altitudes appear veiled by a bluish haze , even on a clear day . When photographed without a filter this haze hides some of the detail visible in the original scene . The detail becomes lost because the image of the haze between the scene and the camera is superimposed upon the image of the distant detail . True atmospheric haze is bluish and is caused by light being scattered by very small particles of dust , water vapor , and , to some extent by the air itself . The blue color of the sky is principally due to the scattering of blue light by the molecules of the air . In outer space beyond the earth's atmosphere , the sky is black .
Atmospheric haze is predominantly ultraviolet with some deep blue , very little green , and even less yellow and red . Since all photographic films are highly sensitive to the invisible ultraviolet as well as to the blue , unfiltered pictures of distant landscapes record more haze than is visible to the eye . If a filter is used to absorb all the light except red , orange , and possibly yellow , the amount of apparent haze diminishes . The deeper the filter color - that is , the farther toward the red - the less haze . For maximum clarity and minimum haze in extremely distant views , infrared - sensitive films are used with infrared filters . Such filters are so deep a red that they appear black . The effectiveness of an infrared filter in penetrating haze and showing up detail in the far distance is shown in Figure 15 .
Figure 15 Infrared film and infrared filter were used to bring out maximum detail in extremely distant mountains .
Courtesy of Kodak Data Book , " Infrared and Ultra - Violet Photography , "
When using a filter to cut through haze , you must not overlook its effect on color contrast . For example , although a red filter penetrates haze very well , it may not be practical to use in some landscapes where true color rendition is necessary . This is because the red filter will make the blue sky photograph black . In such cases , a deep yellow filter , such as the G , may be preferable . Another point to be considered is that as you cut the haze , you bring the distant view closer and lose the feeling of great distance . This may reduce the effectiveness of the picture even if it does increase detail .
Atmospheric haze should not be confused with mist or fog . Both mist and fog consist of actual droplets of water . These droplets are very large compared to the exceedingly fine particles that create haze . Mist and fog droplets scatter all light equally , and for that reason color filters are ineffective in penetrating them .
NEUTRAL DENSITY CONTROL . A neutral density filter is not , strictly speaking , a color filter , since it does not absorb or transmit any one color to a greater degree than any other . It has no color of its own because it absorbs all colors equally , acting in much the same way for film as a pair of neutral sunglasses does for the eyes . It is used primarily to reduce exposure when the more conventional methods are not practical . For example , when motion pictures are to be made in very strong daylight with fast film , the minimum aperture of the movie camera lens may still result in overexposure . In such cases the neutral density filter will do the trick .
In a pinch , a polarizing screen such as the Polaroid filter or the Pola - Screen may be used as a neutral density filter . A more detailed discussion of polarizing filters is given later in this Assignment .
التصوير بالألوان وحقائق حول المرشحات
الجزء الثاني حقائق حول المرشحات
أنواع المرشحات
TYPES OF FILTERS
Color filters for black - and - white photography can serve a number of different purposes , and it is well for the student photographer to bear them clearly in mind . Filters for these purposes may be classified as follows :
1. Correction filters
2. Contrast filters
3. Haze filters
4. Neutral density filters
CORRECTION FILTERS . While panchromatic emulsions respond to all the colors the eye can see , they do not respond to these colors with quite the same relative values as does the eye . For example , blue normally appears darker to the
eye than green , yet a film is very sensitive to blue , and will record it as lighter than green .
By the use of suitable filters , the response of a film can be changed to reproduce colors in the same relative brightness as the eye sees them . For example , to overcome the excess sensitivity of a film to blue , it is necessary to use a filter which absorbs some of the blue . Such a filter appears light yellow . The very popular K2 is such a correction filter for regular panchromatic film . Some of the very fast panchromatic emulsions are excessively sensitive to red as well as to blue . To compensate for such excess red sensitivity , it is necessary to use a filter which absorbs not only some blue but also some red . Such a filter will appear light green . A suitable filter is the Wratten X1 .
In dealing with correction filters , the terms overcorrected and undercorrected are often used . Undercorrected means either that no filter was used , or that the filter was too light in color to fully correct the film to match the color response of the average eye . In an undercorrected landscape photograph , for example , the blue sky is too light , the green grass too dark . Overcorrection is the other extreme , and indicates that a strong filter was used to reverse the normal sensitivity of the film to the color in question . An overcorrected picture would have a very dark sky , as in Figure 11 .
Figure 11 Red filter was used to produce this overcorrected black sky . Photographed by S. J. Silverstein
CONTRAST FILTERS . Contrast filters are used to lighten or darken certain colors in the subject for the purpose of introducing a brightness difference between two colors . This means that one of the colors is made to photograph much lighter or darker than the other , thus increasing their relative contrast .
To the eye , objects are distinguished from their surroundings by two kinds of contrast - tone contrast and color contrast . A black - and - white photograph has only one kind of contrast - tone contrast . It is sometimes necessary to translate color contrast into tone contrast when taking a picture of a colored object . To achieve a specific purpose , the tone contrast may have to be subdued or exaggerated as desired . For example , a red apple and its green leaves may photograph as two nearly identical tones of gray . The color contrast - green against red - disappears in a black - and - white photograph , and only the tone contrast remains . The tone contrast in this case in quite low .
This is clearly shown in Figure 12 , which is a photograph of two plastic tumblers . The upright tumbler is green - the horizontal tumbler is red . The colors were carefully chosen so they have the same visual brightness . As a result the two tumblers photograph to the same shade of gray .
Figu 12 Green upright tumbler and red horizontal tum bler appear same tone of gray because both colors have same visual brightness .
To achieve a tone contrast that more nearly approaches the color contrast as it appears to the eye , one of the two tumblers should be made lighter than the other . If we use a red filter ( Figure 13 ) , the red ( horizontal ) tumbler will appear lighter , and the upright tumbler darker . This is because the red filter lets the red light reflected from the horizontal tumbler through with little interference , but absorbs a high proportion of the green light . Thus the original visual contrast between the two tumblers is restored . The same result , insofar as contrast is concerned , would be achieved by using a green filter ( Figure 14 ) . With this filter the green upright tumbler would become light and the red horizontal tumbler dark . Which of the two filters should be used is a question for the photographer to answer on the basis of individual circumstances .
The student who wishes to understand the use of contrast filters should never forget the following three basic principles :
1. A colored object photographs light if the film is exposed through a filter of the same color as the object itself . For example , consider photographing a Figure 13 Same as Figure 12 , but photographed through red filter Horizontal ( red ) tumbler appears lighter .
Figure 14 Same as Figure 12 , but photographed through green filter . Now . vertical ( green tumbler . appears lighter .
yellowed document . If you used a G ( deep yellow ) filter , which transmits yellow freely , the paper would be reproduced as white .
2. A colored object photographs dark if the negative is exposed through a filter which absorbs the color of the object . Such a filter allows only a little of the light reflected from the object to reach the film so it photographs quite dark . This principle can be used to advantage in photographing a document written in light blue ink . The blue should be made as dark as possible . Because a red filter absorbs blue light , the best contrast is obtained with a red filter . A deep yellow filter , such as the G , also gives good results except for very light blue ink . Also , its filter factor is less . For this reason you may prefer to use a G filter .
3. To show the best detail within a colored object , photograph it so it appears light . Thus dark green leaves show most detail when photographed
through a green filter . The grain pattern of a beautiful piece of red mahogany shows up best if photographed through a red filter . Table IV , page 38 , shows the transmission and absorption characteristics of various color filters .
HAZE FILTERS . Distant landscapes and aerial views from high altitudes appear veiled by a bluish haze , even on a clear day . When photographed without a filter this haze hides some of the detail visible in the original scene . The detail becomes lost because the image of the haze between the scene and the camera is superimposed upon the image of the distant detail . True atmospheric haze is bluish and is caused by light being scattered by very small particles of dust , water vapor , and , to some extent by the air itself . The blue color of the sky is principally due to the scattering of blue light by the molecules of the air . In outer space beyond the earth's atmosphere , the sky is black .
Atmospheric haze is predominantly ultraviolet with some deep blue , very little green , and even less yellow and red . Since all photographic films are highly sensitive to the invisible ultraviolet as well as to the blue , unfiltered pictures of distant landscapes record more haze than is visible to the eye . If a filter is used to absorb all the light except red , orange , and possibly yellow , the amount of apparent haze diminishes . The deeper the filter color - that is , the farther toward the red - the less haze . For maximum clarity and minimum haze in extremely distant views , infrared - sensitive films are used with infrared filters . Such filters are so deep a red that they appear black . The effectiveness of an infrared filter in penetrating haze and showing up detail in the far distance is shown in Figure 15 .
Figure 15 Infrared film and infrared filter were used to bring out maximum detail in extremely distant mountains .
Courtesy of Kodak Data Book , " Infrared and Ultra - Violet Photography , "
When using a filter to cut through haze , you must not overlook its effect on color contrast . For example , although a red filter penetrates haze very well , it may not be practical to use in some landscapes where true color rendition is necessary . This is because the red filter will make the blue sky photograph black . In such cases , a deep yellow filter , such as the G , may be preferable . Another point to be considered is that as you cut the haze , you bring the distant view closer and lose the feeling of great distance . This may reduce the effectiveness of the picture even if it does increase detail .
Atmospheric haze should not be confused with mist or fog . Both mist and fog consist of actual droplets of water . These droplets are very large compared to the exceedingly fine particles that create haze . Mist and fog droplets scatter all light equally , and for that reason color filters are ineffective in penetrating them .
NEUTRAL DENSITY CONTROL . A neutral density filter is not , strictly speaking , a color filter , since it does not absorb or transmit any one color to a greater degree than any other . It has no color of its own because it absorbs all colors equally , acting in much the same way for film as a pair of neutral sunglasses does for the eyes . It is used primarily to reduce exposure when the more conventional methods are not practical . For example , when motion pictures are to be made in very strong daylight with fast film , the minimum aperture of the movie camera lens may still result in overexposure . In such cases the neutral density filter will do the trick .
In a pinch , a polarizing screen such as the Polaroid filter or the Pola - Screen may be used as a neutral density filter . A more detailed discussion of polarizing filters is given later in this Assignment .
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