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التصوير الفوتوغرافي الحديث
طرق الحفاظ على الجودة
تصنيف سرعة الفيلم والتعرض
FILM SPEED RATING AND EXPOSURE
The question of film speed is perhaps the most puzzling aspect of available light photography for most students . Everyone has heard stories of the fantastic film speeds used by some photographers - speeds which bear no resemblance to the manufacturer's published data . Is there some souped - up developer or some highly guarded secret of processing that makes this possible ? The answer is emphatically no ! As we pointed out earlier in this Assignment , every time you increase the speed rating of a film , you are deliberately underexposing it . Within limits , such underexposure may still produce fairly satisfactory results . Those limits may be wide indeed if you stretch the meaning of the word " satisfactory " correspondingly . But never lose sight of the fact that the more you push a film , the less likely you are to get a good picture .
Figure 25 Art gallery interior , illuminated by large bank of overhead fluorescent lamps . Courtesy of J. Oppenheim
The first step toward correct exposure is to set your meter to a realistic exposure index ( speed rating ) . As was mentioned earlier , whenever possible , this should be the manufacturer's rated speed . At first , film manufacturers put a 2.5 times safety factor into their ratings for all black - and - white films . The purpose was to guard against underexposure on the theory that the average amateur couldn't be trusted to use a meter correctly and it was safer to overexpose than to underexpose . Under this system , a film rated at ASA 80 really had a speed of ASA 200 .
The knowledgeable photographer soon discovered he could double the speed rating of his film and get better negatives . This caused a considerable amount of confusion , particularly since no such generous margin for error was added to the speed rating of color film . Eventually , to clear up the confusion , the ratings of black - and - white films were doubled by their manufacturers , so that a film which had been rated at 80 was re - rated to 160. The actual speed of the film , of course , remained the same . The true speed of this film , you must remember , is 200 , but the 25 % safety factor obtained by rating it at 160 is desirable for general use . However , if the lighting is not too bright and you need all the speed you can get , you can neglect the 25 % safety factor and rate this film at ASA 200. This will give you correct exposure throughout the normal brightness range of 128 to 1 .
We can now set up some general rules of exposure for taking available light pictures . For average use , follow the manufacturer's speed rating . The 25 % margin cannot hurt your negatives , and it protect against minor errors in reading the meter or the use of incorrectly calibrated lenses , shutters and exposure meters . If you need the extra speed , increase the rating by 25 % ( from 160 to 200 , for example ) . This is still correct exposure and will give good results for highlights , middle tones and normal shadows . If the lighting is contrasty and the shadows are quite dark , they may lose some detail .
Figure 26 Modern office building foyer daylight illumination ..
Figure 27 Same as Figure 26 , with left side printed - in to bring out details in street beyond .
Figure 28 Museum interior , illuminated by light coming through window at right , just outside picture area . Courtesy of G. Maling
In an emergency you can double the maximum speed , shooting at 400 for a 160 rated film or 1000 for a 400 rated film . In effect , you are now treating your highlights as though they are middle tones , and your middle tones as shadows . They will , therefore , come through quite well . Your shadows may or may not , depending upon the lighting . If the lighting is contrasty and the shadows quite dark , you will lose most of the detail ; if the lighting is quite flat , with no deep shadows , you will retain a good part of the detail .
When the film is up - rated this way , you may have to increase development time , since underexposure reduces the negative contrast to some extent . How much increase is necessary depends on the developer and the film . Follow the manufacturer's instructions . However , a 25 % increase in developing time is a good starting point if you haven't pushed the speed too much - 50 % may be better for extreme underexposure . Generally , the more rapid , more contrasty developers continue to build up density at a good rate after the normal development time has elapsed . Such developers require less increase in time to build density in underexposed negatives ..
You can get a good idea of how your shadows will come through by taking meter readings in both shadows and highlights , and seeing whether they come within the U and O markings of your meter . If your meter is not so marked , take readings of both shadows and highlights in the usual way . If the shadows read within 7 stops of the highlights for black - and - white and color negative film or 3 stops for color reversal film , they will photograph reasonably well . If there is a greater difference between shadow and highlights than the above figures , you will have to reconcile yourself to empty shadows . This method of determining lighting ratio and correct exposure was discussed in more detail in the Assignment on exposure meters . Be sure to re - read it at this point .
طرق الحفاظ على الجودة
تصنيف سرعة الفيلم والتعرض
FILM SPEED RATING AND EXPOSURE
The question of film speed is perhaps the most puzzling aspect of available light photography for most students . Everyone has heard stories of the fantastic film speeds used by some photographers - speeds which bear no resemblance to the manufacturer's published data . Is there some souped - up developer or some highly guarded secret of processing that makes this possible ? The answer is emphatically no ! As we pointed out earlier in this Assignment , every time you increase the speed rating of a film , you are deliberately underexposing it . Within limits , such underexposure may still produce fairly satisfactory results . Those limits may be wide indeed if you stretch the meaning of the word " satisfactory " correspondingly . But never lose sight of the fact that the more you push a film , the less likely you are to get a good picture .
Figure 25 Art gallery interior , illuminated by large bank of overhead fluorescent lamps . Courtesy of J. Oppenheim
The first step toward correct exposure is to set your meter to a realistic exposure index ( speed rating ) . As was mentioned earlier , whenever possible , this should be the manufacturer's rated speed . At first , film manufacturers put a 2.5 times safety factor into their ratings for all black - and - white films . The purpose was to guard against underexposure on the theory that the average amateur couldn't be trusted to use a meter correctly and it was safer to overexpose than to underexpose . Under this system , a film rated at ASA 80 really had a speed of ASA 200 .
The knowledgeable photographer soon discovered he could double the speed rating of his film and get better negatives . This caused a considerable amount of confusion , particularly since no such generous margin for error was added to the speed rating of color film . Eventually , to clear up the confusion , the ratings of black - and - white films were doubled by their manufacturers , so that a film which had been rated at 80 was re - rated to 160. The actual speed of the film , of course , remained the same . The true speed of this film , you must remember , is 200 , but the 25 % safety factor obtained by rating it at 160 is desirable for general use . However , if the lighting is not too bright and you need all the speed you can get , you can neglect the 25 % safety factor and rate this film at ASA 200. This will give you correct exposure throughout the normal brightness range of 128 to 1 .
We can now set up some general rules of exposure for taking available light pictures . For average use , follow the manufacturer's speed rating . The 25 % margin cannot hurt your negatives , and it protect against minor errors in reading the meter or the use of incorrectly calibrated lenses , shutters and exposure meters . If you need the extra speed , increase the rating by 25 % ( from 160 to 200 , for example ) . This is still correct exposure and will give good results for highlights , middle tones and normal shadows . If the lighting is contrasty and the shadows are quite dark , they may lose some detail .
Figure 26 Modern office building foyer daylight illumination ..
Figure 27 Same as Figure 26 , with left side printed - in to bring out details in street beyond .
Figure 28 Museum interior , illuminated by light coming through window at right , just outside picture area . Courtesy of G. Maling
In an emergency you can double the maximum speed , shooting at 400 for a 160 rated film or 1000 for a 400 rated film . In effect , you are now treating your highlights as though they are middle tones , and your middle tones as shadows . They will , therefore , come through quite well . Your shadows may or may not , depending upon the lighting . If the lighting is contrasty and the shadows quite dark , you will lose most of the detail ; if the lighting is quite flat , with no deep shadows , you will retain a good part of the detail .
When the film is up - rated this way , you may have to increase development time , since underexposure reduces the negative contrast to some extent . How much increase is necessary depends on the developer and the film . Follow the manufacturer's instructions . However , a 25 % increase in developing time is a good starting point if you haven't pushed the speed too much - 50 % may be better for extreme underexposure . Generally , the more rapid , more contrasty developers continue to build up density at a good rate after the normal development time has elapsed . Such developers require less increase in time to build density in underexposed negatives ..
You can get a good idea of how your shadows will come through by taking meter readings in both shadows and highlights , and seeing whether they come within the U and O markings of your meter . If your meter is not so marked , take readings of both shadows and highlights in the usual way . If the shadows read within 7 stops of the highlights for black - and - white and color negative film or 3 stops for color reversal film , they will photograph reasonably well . If there is a greater difference between shadow and highlights than the above figures , you will have to reconcile yourself to empty shadows . This method of determining lighting ratio and correct exposure was discussed in more detail in the Assignment on exposure meters . Be sure to re - read it at this point .
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