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دقة الألوان .. اختيار الفيلم .. كيفية التقاط وتطوير الصور الملونة
COLOUR ACCURACY
In looking at different brands of film , we have already touched on the problems of reproducing colours accurately . What makes colour photography possible at all is the convenient fact that three primary colours , mixed in the right proportion , give a fair approximation of most other colours . They can even give an exact rendering of some , but not of all at the same time , at least not with the current state of the art in film technology .
Most of the time , colour accuracy is not an issue . It does become important with two kinds of colour : a group of hues to which photographic film behaves strangely , and what are known as recognition colours .
The first group includes dyes and surfaces that reflect strongly ( but invisibly ) in ultra - violet , far red and infra - red . Examples are blue morning glory flowers , and organic fabric dyes , particularly in dark green textiles .
The second set of hues , recognition colours , are those that the eye dis- criminates more sensitively than it does any others . They include grey , which is really an absence of colour , and very familiar ones , such as skin tones and the blue of a cloudless sky . Even a slight departure from an accurate version instantly looks wrong . A painted barn could be virtually any colour at all in a photograph , yet a viewer would know if it were accurate only by holding the picture up against the real barn . A person's face , on the other hand , is an entirely different case : a pale complexion is already a delicate blend of hues , so that just a touch extra of green , say , or yellow , can make the subject of a portrait look ill !
Colour casts can be a particular problem with portraits . where a person can appear positively ill once the slightest shade of yellow or green ( above ) is introduced ! Here , it is important that the film is not out of date , and that light sources that may cause unpleasant casts ( such as fluorescent lights ) are kept well away .
ABOVE The pale image , weak shadows and green colour cast typical of film used long after the expiry date marked on its box .
These are the inherent problems of colour photography that the film maker has to face , and generally they manage well enough . The main reasons why a photographer occasionally finds a colour cast over the image are that the film may be old , stored badly , or that a long exposure has caused a shift in the colour layers . When film ages , it turns towards one colour , often green , because the three colour layers in the emulsion become less sensitive at different rates . Long exposures cause a similar effect : the reciprocity between aperture and shutter speed ( for instance , increasing one and decreasing the other by the same amount should give the same exposure ) no longer works . The film becomes less sensitive at long exposures , but the three colour layers react slightly differently . Hence there is a colour shift .
LEFT AND ABOVE LEFT The danger with long exposures is that the reciprocity law between shutter speeds and apertures may be broken , resulting in a strong colour cast ( left ) . It is possible to compensate for this , and most manufacturers can supply instructions as to the exact exposure adjustment required .
BELOW AND BELOW LEFT A photo that may have taken ages to set up can often be ruined by the use of out - of- date film Compare the photo taken using good stock ( left ) with the picture displaying the green cast , taken on aged emulsion ( right ) .
COLOUR ACCURACY
In looking at different brands of film , we have already touched on the problems of reproducing colours accurately . What makes colour photography possible at all is the convenient fact that three primary colours , mixed in the right proportion , give a fair approximation of most other colours . They can even give an exact rendering of some , but not of all at the same time , at least not with the current state of the art in film technology .
Most of the time , colour accuracy is not an issue . It does become important with two kinds of colour : a group of hues to which photographic film behaves strangely , and what are known as recognition colours .
The first group includes dyes and surfaces that reflect strongly ( but invisibly ) in ultra - violet , far red and infra - red . Examples are blue morning glory flowers , and organic fabric dyes , particularly in dark green textiles .
The second set of hues , recognition colours , are those that the eye dis- criminates more sensitively than it does any others . They include grey , which is really an absence of colour , and very familiar ones , such as skin tones and the blue of a cloudless sky . Even a slight departure from an accurate version instantly looks wrong . A painted barn could be virtually any colour at all in a photograph , yet a viewer would know if it were accurate only by holding the picture up against the real barn . A person's face , on the other hand , is an entirely different case : a pale complexion is already a delicate blend of hues , so that just a touch extra of green , say , or yellow , can make the subject of a portrait look ill !
Colour casts can be a particular problem with portraits . where a person can appear positively ill once the slightest shade of yellow or green ( above ) is introduced ! Here , it is important that the film is not out of date , and that light sources that may cause unpleasant casts ( such as fluorescent lights ) are kept well away .
ABOVE The pale image , weak shadows and green colour cast typical of film used long after the expiry date marked on its box .
These are the inherent problems of colour photography that the film maker has to face , and generally they manage well enough . The main reasons why a photographer occasionally finds a colour cast over the image are that the film may be old , stored badly , or that a long exposure has caused a shift in the colour layers . When film ages , it turns towards one colour , often green , because the three colour layers in the emulsion become less sensitive at different rates . Long exposures cause a similar effect : the reciprocity between aperture and shutter speed ( for instance , increasing one and decreasing the other by the same amount should give the same exposure ) no longer works . The film becomes less sensitive at long exposures , but the three colour layers react slightly differently . Hence there is a colour shift .
LEFT AND ABOVE LEFT The danger with long exposures is that the reciprocity law between shutter speeds and apertures may be broken , resulting in a strong colour cast ( left ) . It is possible to compensate for this , and most manufacturers can supply instructions as to the exact exposure adjustment required .
BELOW AND BELOW LEFT A photo that may have taken ages to set up can often be ruined by the use of out - of- date film Compare the photo taken using good stock ( left ) with the picture displaying the green cast , taken on aged emulsion ( right ) .
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