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البطيء مقابل السريع .. اختيار الفيلم .. كيفية التقاط وتطوير الصور بالأبيض والأسود
SLOW VERSUS FAST
As we have just seen , choosing a black - and - white film means deciding on prior- ities , and the principal choice is between film speed and graininess . Were it possible to discount the effects of grain , there would be no need for different film speeds . As it is , there is a range , the fastest being several times more light- sensitive than the slowest .
Film speed depends mainly on the size and shape of the crystals in the emulsion . A large crystal exposed to the same amount of light as a small crystal can be developed just as easily to black silver , and is more visible because of its size . Also , if the grains are flat rather than lumpy , as in some of the latest films , and if these grains are aligned so that the flat side faces the light , they will react more readily . The amount of development and the chemistry of the developer can also be used to make film behave as if it were faster .
In practice , the sensitivity of film is referred to in terms of its speed . An average emulsion - medium - speed , in other words is one that allows the camera to be used conveniently in most natural lighting . This means a shutter speed of around 1/125 sec and an aperture setting somewhere in the middle of the range on the lens . This is a fairly vague definition , and in order to be able to calibrate the camera controls and make accurate exposures , film speed has a standard scale to which all makes now conform . The universally accepted
measurement is the ISO number . ISO stands for International Standards Organisation , and the rating is made up of two numbers . The first is the equivalent of the old ASA figure , the second is equivalent to the system used mainly in Germany : DIN .
A medium - speed film has an ISO rating of between 100 / 21 ° and 125 / 22 ° . In normal practice , the second figure means so little outside Germany that it is normally dropped .
Film speeds have become standardized over the years , and there are now just three main categories : slow , medium and fast . Slow films are between ISO25 and 32 , medium between ISO100 and 125 , and fast at ISO400 . In addition , there are some specialized black - and - white emulsions with more extreme rat- ings – for instance , high - contrast lith films rated at around ISO12 and surveil- lance films rated over ISO1000 .
A fast film is more convenient to use in practically every way than a slow film . It can be used in poorer light , allows a faster shutter speed ( and so less chance of a blurred image through camera shake ) and a smaller aperture ( for better depth of field ) . The only circumstances in which this might not be wanted are if you need a slow shutter speed with shallow depth of field . Nevertheless , choos- ing the film speed involves other factors , notably graininess , which we deal with in detail next .
BELOW Using fast film does not always mean an unsatisfactory graininess as this example shows . Such a photograph would be impossible on slow film .
BELOW Certain scenes , such as this still - life set , demand that you use a film capable of rendering as much detail as possible . Because of their very fine grain , slow films - - between ISO25 and ISO32 - are by far the best choice .
BELOW LEFT On other occasions , you will need to photograph scenes that feature very high contrast ; again , slow films ' ability to handle high contrast makes them perfect for the job .
SLOW VERSUS FAST
As we have just seen , choosing a black - and - white film means deciding on prior- ities , and the principal choice is between film speed and graininess . Were it possible to discount the effects of grain , there would be no need for different film speeds . As it is , there is a range , the fastest being several times more light- sensitive than the slowest .
Film speed depends mainly on the size and shape of the crystals in the emulsion . A large crystal exposed to the same amount of light as a small crystal can be developed just as easily to black silver , and is more visible because of its size . Also , if the grains are flat rather than lumpy , as in some of the latest films , and if these grains are aligned so that the flat side faces the light , they will react more readily . The amount of development and the chemistry of the developer can also be used to make film behave as if it were faster .
In practice , the sensitivity of film is referred to in terms of its speed . An average emulsion - medium - speed , in other words is one that allows the camera to be used conveniently in most natural lighting . This means a shutter speed of around 1/125 sec and an aperture setting somewhere in the middle of the range on the lens . This is a fairly vague definition , and in order to be able to calibrate the camera controls and make accurate exposures , film speed has a standard scale to which all makes now conform . The universally accepted
measurement is the ISO number . ISO stands for International Standards Organisation , and the rating is made up of two numbers . The first is the equivalent of the old ASA figure , the second is equivalent to the system used mainly in Germany : DIN .
A medium - speed film has an ISO rating of between 100 / 21 ° and 125 / 22 ° . In normal practice , the second figure means so little outside Germany that it is normally dropped .
Film speeds have become standardized over the years , and there are now just three main categories : slow , medium and fast . Slow films are between ISO25 and 32 , medium between ISO100 and 125 , and fast at ISO400 . In addition , there are some specialized black - and - white emulsions with more extreme rat- ings – for instance , high - contrast lith films rated at around ISO12 and surveil- lance films rated over ISO1000 .
A fast film is more convenient to use in practically every way than a slow film . It can be used in poorer light , allows a faster shutter speed ( and so less chance of a blurred image through camera shake ) and a smaller aperture ( for better depth of field ) . The only circumstances in which this might not be wanted are if you need a slow shutter speed with shallow depth of field . Nevertheless , choos- ing the film speed involves other factors , notably graininess , which we deal with in detail next .
BELOW Using fast film does not always mean an unsatisfactory graininess as this example shows . Such a photograph would be impossible on slow film .
BELOW Certain scenes , such as this still - life set , demand that you use a film capable of rendering as much detail as possible . Because of their very fine grain , slow films - - between ISO25 and ISO32 - are by far the best choice .
BELOW LEFT On other occasions , you will need to photograph scenes that feature very high contrast ; again , slow films ' ability to handle high contrast makes them perfect for the job .
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