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التصوير الفوتوغرافي الحديث
تحديد الوضع الصحيح ..
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RANGE OF EXPOSURE
If you think about it، it is easy to understand that a film never gets one single exposure . The actual exposure of any given portion of the film is determined by brightness of the image projected on it by the lens at that particular point . The average film , therefore , receives a wide range of exposures . This exposure range depends on the range of brightness of the subject . The correct overall exposure is that exposure which makes it possible for the film to record the full range of tones , from the brightest to the darkest .
The average black - and - white film has the ability to record accurately a brightness range of about 128 to 1. This means that if the darkest area in a subject is considered to have a brightness value of , and the lightest area has a brightness value of 128 , the film can accurately record all the tones of this scene . In actual practice , the average subject seldom has a brightness range as wide as this . If we take an average landscape , for example , and measure the amount of light reflected from each part of it , we may find the brightest area only 32 times as bright as the darkest areas . If we photograph this scene , using an exposure which will adequately expose the shadows , the highlights will produce only 32 times as much effect on the film . This range of 32 to 1 is well within the film's exposure range of 128 to 1 and we will get a good negative .
Let us photograph the same scene again , but give it four times the first exposure by opening the lens two stops . The shadows will have an exposure value of 4 and the highlights 4 x 32 , or 128. This will still be within the brightness range of the film , and we will again get a good negative . The shadows in this second negative will be noticeably denser than those of the first negative , and it will take longer to print , but prints from both negatives will be practically identical . We say , therefore , that the exposure latitude of this film is 4 because we can increase the exposure of the film four times and still produce a good negative .
The obvious question arises - which is the better negative ? For small prints , the darker , denser negative might be better . With the thinner negative , any slight scratch in handling would show in the final print . However , if the negative is to be greatly enlarged , particularly if it is 35mm , the thinner negative is better . It will need less printing time , show less grain in the finished print , and tend to give . somewhat greater contrast to the picture when it is printed with a condenser - type enlarger . All these factors will be discussed at appropriate points in future Assignments .
In our example of the landscape photograph , you saw that exposure could be increased four times without lowering the quality of the negative . Such exposure latitude for black - and - white film is not at all unusual for most subjects . You can frequently stretch exposure latitude to as much as 10 or even 20 to 1 if you are prepared to sacrifice some detail in either the shadows or extreme highlights . For example , if you are not interested in showing any detail in the shadows , you can underexpose slightly by cutting the exposure in half without hurting the picture to any great extent . On the other hand , let us suppose the only highlights in the picture are some light patches of sky and reflections from water . If the sky is quite clear and cloudless , we don't have to worry about reproducing its tone accurately . You can overexpose slightly by doubling the maximum correct exposure .
The picture in Figure 1 illustrates the kind of subject which permits a rather serious error in exposure without affecting print quality too much . All the interest lies in the middle tones , which have a relatively narrow range of brightness . The shadows are small and unimportant , so they can go black without harm . The strongest highlights have no detail and the sky is cloudless , so tonal differentiation is unimportant here . The picture in Figure 2 , on the other hand , requires absolute accuracy in exposure . Even slight underexposure will produce
undesirable empty shadows . Overexposure will kill all the delicate highlights and lose the interesting luminous quality of the candles and smoke . Portraits also generally call for very accurate exposure .
Fig . 1 A subject in which serious error in exposure may be tolerated . There are no deep shadows and highlights have little detail .
Fig . 2 This subject requires extremely accu rate exposure . Overexposure will lose the modeling in the candle . Underexposure will lose the delicacy in the smoke .
تحديد الوضع الصحيح ..
مدى التعرض ..
RANGE OF EXPOSURE
If you think about it، it is easy to understand that a film never gets one single exposure . The actual exposure of any given portion of the film is determined by brightness of the image projected on it by the lens at that particular point . The average film , therefore , receives a wide range of exposures . This exposure range depends on the range of brightness of the subject . The correct overall exposure is that exposure which makes it possible for the film to record the full range of tones , from the brightest to the darkest .
The average black - and - white film has the ability to record accurately a brightness range of about 128 to 1. This means that if the darkest area in a subject is considered to have a brightness value of , and the lightest area has a brightness value of 128 , the film can accurately record all the tones of this scene . In actual practice , the average subject seldom has a brightness range as wide as this . If we take an average landscape , for example , and measure the amount of light reflected from each part of it , we may find the brightest area only 32 times as bright as the darkest areas . If we photograph this scene , using an exposure which will adequately expose the shadows , the highlights will produce only 32 times as much effect on the film . This range of 32 to 1 is well within the film's exposure range of 128 to 1 and we will get a good negative .
Let us photograph the same scene again , but give it four times the first exposure by opening the lens two stops . The shadows will have an exposure value of 4 and the highlights 4 x 32 , or 128. This will still be within the brightness range of the film , and we will again get a good negative . The shadows in this second negative will be noticeably denser than those of the first negative , and it will take longer to print , but prints from both negatives will be practically identical . We say , therefore , that the exposure latitude of this film is 4 because we can increase the exposure of the film four times and still produce a good negative .
The obvious question arises - which is the better negative ? For small prints , the darker , denser negative might be better . With the thinner negative , any slight scratch in handling would show in the final print . However , if the negative is to be greatly enlarged , particularly if it is 35mm , the thinner negative is better . It will need less printing time , show less grain in the finished print , and tend to give . somewhat greater contrast to the picture when it is printed with a condenser - type enlarger . All these factors will be discussed at appropriate points in future Assignments .
In our example of the landscape photograph , you saw that exposure could be increased four times without lowering the quality of the negative . Such exposure latitude for black - and - white film is not at all unusual for most subjects . You can frequently stretch exposure latitude to as much as 10 or even 20 to 1 if you are prepared to sacrifice some detail in either the shadows or extreme highlights . For example , if you are not interested in showing any detail in the shadows , you can underexpose slightly by cutting the exposure in half without hurting the picture to any great extent . On the other hand , let us suppose the only highlights in the picture are some light patches of sky and reflections from water . If the sky is quite clear and cloudless , we don't have to worry about reproducing its tone accurately . You can overexpose slightly by doubling the maximum correct exposure .
The picture in Figure 1 illustrates the kind of subject which permits a rather serious error in exposure without affecting print quality too much . All the interest lies in the middle tones , which have a relatively narrow range of brightness . The shadows are small and unimportant , so they can go black without harm . The strongest highlights have no detail and the sky is cloudless , so tonal differentiation is unimportant here . The picture in Figure 2 , on the other hand , requires absolute accuracy in exposure . Even slight underexposure will produce
undesirable empty shadows . Overexposure will kill all the delicate highlights and lose the interesting luminous quality of the candles and smoke . Portraits also generally call for very accurate exposure .
Fig . 1 A subject in which serious error in exposure may be tolerated . There are no deep shadows and highlights have little detail .
Fig . 2 This subject requires extremely accu rate exposure . Overexposure will lose the modeling in the candle . Underexposure will lose the delicacy in the smoke .
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