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حساسية اللون .. اختيار الفيلم .. كيفية التقاط وتطوير الصور بالأبيض والأسود
COLOUR SENSITIVITY
Colour depends on the wavelength of light and on the way in which the human brain has become accustomed to interpret it . Redness or blueness are simply the ways in which we recognize specific wavelengths .
Black - and - white film does not actually show colour in this familiar way , as different hues , but it does record all the same wavelengths . A black - and - white print then displays the colours as different tones of grey ; but as the originals can also be bright or dark , most original colours can turn out in any shade .
This , however , only causes confusion when the object is unfamiliar . With something obvious , memory fills the gap - we simply assume , for instance , that grass in a monochrome photograph is green , however dark or light grey it looks . One of the special advantages of black - and - white photography is that the darkness and lightness of coloured objects can be altered with filters .
In normal ways , however , black - and - white film has a particular response to different wavelengths , and it is not perfect . Neither , however , is the way our eyes see , and what film manufacturers have tried to do , by using special dyes in the emulsions as filters , is to match the response of human vision .
Light is simply visible wavelengths ; it is , in other words what we can see . There is much more radiation shorter and longer than the wavelengths repre- sented by colours : ultra - violet is shorter than blue and violet , infrared is longer than red , and both are invisible . Our eyes have the best response in the middle of the scale of colours , to yellow - green , and the least to violet and deep red .
The silver halide crystals that form the basis of all film emulsions unfortu- nately have a different response . Used without special treatment they are very sensitive to blue and hardly react at all to red . In the darkroom , when printing , this is helpful , because the paper can be handled perfectly safely in a red light bright enough for comfortable vision . When shooting scenes with a camera , on the other hand , this would be a serious problem , upsetting the expected tonal balance . Indeed , in the early days of photography , it was such a problem that white skies and dark flesh tones were typical . Clouds in a blue sky were ren- dered virtually invisible , because the blue exposed so strongly , to the point where it became common practice to add skies to a photograph from another negative .
Modern black - and - white film overcomes most of this with built - in filters to even out this sensitivity , and so is called ' panchromatic ' . Still , some of the old skewed response remains , and blue skies still appear paler than visual memory . Some special emulsions that are intended to be used under red darkroom safelighting , such as lith films , are deliberately left insensitive to red - they are called ' orthochromatic ' .
ABOVE Silver halide crystals are very sensitive to blue and hardly react at all to red without special treatment . In the early days of photography , skies used to be added to pictures from other negatives , although these days the problem can be resolved with filters .
FAR LEFT Without special treatment , black - and - white film usually renders blue skies far blander than we remember them .
LEFT However , the
situation can be improv with the use of an orange or yellow filter fitted to the lens . The choice of filter depends upon the strength of sky required ; orange will give a dramatic , deep sky , while yellow will give a more ' realistic ' rendition .
COLOUR SENSITIVITY
Colour depends on the wavelength of light and on the way in which the human brain has become accustomed to interpret it . Redness or blueness are simply the ways in which we recognize specific wavelengths .
Black - and - white film does not actually show colour in this familiar way , as different hues , but it does record all the same wavelengths . A black - and - white print then displays the colours as different tones of grey ; but as the originals can also be bright or dark , most original colours can turn out in any shade .
This , however , only causes confusion when the object is unfamiliar . With something obvious , memory fills the gap - we simply assume , for instance , that grass in a monochrome photograph is green , however dark or light grey it looks . One of the special advantages of black - and - white photography is that the darkness and lightness of coloured objects can be altered with filters .
In normal ways , however , black - and - white film has a particular response to different wavelengths , and it is not perfect . Neither , however , is the way our eyes see , and what film manufacturers have tried to do , by using special dyes in the emulsions as filters , is to match the response of human vision .
Light is simply visible wavelengths ; it is , in other words what we can see . There is much more radiation shorter and longer than the wavelengths repre- sented by colours : ultra - violet is shorter than blue and violet , infrared is longer than red , and both are invisible . Our eyes have the best response in the middle of the scale of colours , to yellow - green , and the least to violet and deep red .
The silver halide crystals that form the basis of all film emulsions unfortu- nately have a different response . Used without special treatment they are very sensitive to blue and hardly react at all to red . In the darkroom , when printing , this is helpful , because the paper can be handled perfectly safely in a red light bright enough for comfortable vision . When shooting scenes with a camera , on the other hand , this would be a serious problem , upsetting the expected tonal balance . Indeed , in the early days of photography , it was such a problem that white skies and dark flesh tones were typical . Clouds in a blue sky were ren- dered virtually invisible , because the blue exposed so strongly , to the point where it became common practice to add skies to a photograph from another negative .
Modern black - and - white film overcomes most of this with built - in filters to even out this sensitivity , and so is called ' panchromatic ' . Still , some of the old skewed response remains , and blue skies still appear paler than visual memory . Some special emulsions that are intended to be used under red darkroom safelighting , such as lith films , are deliberately left insensitive to red - they are called ' orthochromatic ' .
ABOVE Silver halide crystals are very sensitive to blue and hardly react at all to red without special treatment . In the early days of photography , skies used to be added to pictures from other negatives , although these days the problem can be resolved with filters .
FAR LEFT Without special treatment , black - and - white film usually renders blue skies far blander than we remember them .
LEFT However , the
situation can be improv with the use of an orange or yellow filter fitted to the lens . The choice of filter depends upon the strength of sky required ; orange will give a dramatic , deep sky , while yellow will give a more ' realistic ' rendition .
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