تويت
التصوير الفوتوغرافي الحديث
الجزء الأول - تصوير فلاش
تحديد التعرض للمصابيح
DETERMINING EXPOSURE FOR FLASHBULBS
There is no practical way for a photographer to take a meter reading of the light produced by a flashbulb . He must judge correct exposure through exposure tables or calculators . Flash lamp manufacturers issue exposure guide numbers with their flash lamps . These guide numbers make it possible to determine correct exposure by using an exposure table which takes into consideration film speed , lamp - to - subject distance and shutter time . Guide numbers for the more common flashbulbs are given in Table I.
The guide number table gives the exposure guide number of each flashbulb for any combination of film and shutter speeds . The only remaining factor to be determined is the lens opening . This is obtained by dividing the guide number by the distance in feet between the flash and the subject . For example , let us say you are using a 5 flashbulb with a film having an ASA rating of 125 , and you want to give an exposure of 1/125 second to stop motion . Run down the first column until you come to the 5 flashbulb .
The next item is the shutter speed , so select 1/125 second and follow this line across the page until you meet the column headed ASA 125. The number in this space is 250 , the guide number for this combination of flashbulb , shutter speed and film speed . You simply divide this guide number by the distance in feet from your subject and you have the f / number . For example , if your subject is 12 feet away , divide 250 by 12 , which gives you 20.8 , or f / 22 , which is the closest f / stop .
At this point we cannot overemphasize the importance of accurately esti mating the distance between subject and flash lamp , particularly for color work . The brightness of the light reaching any subject decreases as the square of the distance . As an example , if you have calculated the exposure for 8 feet , and later move back to 16 feet to include more in the picture , you must give four times the exposure , which is the same as half the f / number . This means opening the lens two full stops . If you don't , the second picture will be badly underexposed .
It is only too easy to disregard this fundamental law of lighting when you are busy shooting exciting pictures at a party , a wedding reception , or other occasions where there is great activity . However , if you want successful pictures , you must always bear it in mind .
To change exposure from one picture to the next , it is always best to adjust the lens opening rather than the shutter speed . Because of the nature of the illumination curve of a flashbulb ( see Figure 1 ) , a change in shutter speed usually does not result in a proportional change in exposure . For example , although 1/50 second is four times as long as 1/200 second , only 2½ times as much light is admitted from a flashbulb . This is because the 1/200 second exposure picks up only the peak of the flash , while the longer exposure picks up a good part of the light when it is considerably below peak value ..
The guide numbers given by the flash lamp manufacturers are to be used as guides , as the name implies . They are intended for average use with average equipment . They may not work best for you or for your equipment . If you find , for example , that your flash pictures are consistently underexposed , use a half stop larger lens opening or use a correspondingly smaller guide number than the table specifies . On the other hand , if your pictures are consistently overexposed , use a larger guide number or stop the lens down half a stop .
Aside from the efficiency of your equipment , several factors can modify the basic exposure calculations of the table . One very important one is the nature of the surroundings . If the room is small with light walls and a low ceiling , a great deal of light will be reflected from the walls and ceiling . This light will increase overall illumination . You may have to stop down one full stop to avoid overexposure . On the other hand , if you are shooting outdoors or in a large auditorium or gymnasium , there will be no reflection of light to help you . You may have to open up one full stop , or possibly two , to get a properly exposed picture . Always take note of your surroundings and adjust your flash exposure accordingly .
The type of reflector you use also affects the exposure and , therefore , the guide number . Published guide number tables usually specify the type of reflectors for which they were computed . Don't use the table published for a polished , high efficiency reflector if you are using one which is dull , shallow and inefficient . You may have to increase the lens opening one or more stops to get correctly exposed pictures if you use an inefficient reflector .
The third condition you should consider is the nature of your subject . For particularly dark subjects , open up a half stop ; for particularly light subjects , close down a half stop .
Finally , if the picture is important , and you want to be sure of exposing it perfectly , bracket your exposures . Make three exposures , one at what you believe to be the correct exposure , one positive color film , use only a half stop variation because of its limited exposure latitude .
الجزء الأول - تصوير فلاش
تحديد التعرض للمصابيح
DETERMINING EXPOSURE FOR FLASHBULBS
There is no practical way for a photographer to take a meter reading of the light produced by a flashbulb . He must judge correct exposure through exposure tables or calculators . Flash lamp manufacturers issue exposure guide numbers with their flash lamps . These guide numbers make it possible to determine correct exposure by using an exposure table which takes into consideration film speed , lamp - to - subject distance and shutter time . Guide numbers for the more common flashbulbs are given in Table I.
The guide number table gives the exposure guide number of each flashbulb for any combination of film and shutter speeds . The only remaining factor to be determined is the lens opening . This is obtained by dividing the guide number by the distance in feet between the flash and the subject . For example , let us say you are using a 5 flashbulb with a film having an ASA rating of 125 , and you want to give an exposure of 1/125 second to stop motion . Run down the first column until you come to the 5 flashbulb .
The next item is the shutter speed , so select 1/125 second and follow this line across the page until you meet the column headed ASA 125. The number in this space is 250 , the guide number for this combination of flashbulb , shutter speed and film speed . You simply divide this guide number by the distance in feet from your subject and you have the f / number . For example , if your subject is 12 feet away , divide 250 by 12 , which gives you 20.8 , or f / 22 , which is the closest f / stop .
At this point we cannot overemphasize the importance of accurately esti mating the distance between subject and flash lamp , particularly for color work . The brightness of the light reaching any subject decreases as the square of the distance . As an example , if you have calculated the exposure for 8 feet , and later move back to 16 feet to include more in the picture , you must give four times the exposure , which is the same as half the f / number . This means opening the lens two full stops . If you don't , the second picture will be badly underexposed .
It is only too easy to disregard this fundamental law of lighting when you are busy shooting exciting pictures at a party , a wedding reception , or other occasions where there is great activity . However , if you want successful pictures , you must always bear it in mind .
To change exposure from one picture to the next , it is always best to adjust the lens opening rather than the shutter speed . Because of the nature of the illumination curve of a flashbulb ( see Figure 1 ) , a change in shutter speed usually does not result in a proportional change in exposure . For example , although 1/50 second is four times as long as 1/200 second , only 2½ times as much light is admitted from a flashbulb . This is because the 1/200 second exposure picks up only the peak of the flash , while the longer exposure picks up a good part of the light when it is considerably below peak value ..
The guide numbers given by the flash lamp manufacturers are to be used as guides , as the name implies . They are intended for average use with average equipment . They may not work best for you or for your equipment . If you find , for example , that your flash pictures are consistently underexposed , use a half stop larger lens opening or use a correspondingly smaller guide number than the table specifies . On the other hand , if your pictures are consistently overexposed , use a larger guide number or stop the lens down half a stop .
Aside from the efficiency of your equipment , several factors can modify the basic exposure calculations of the table . One very important one is the nature of the surroundings . If the room is small with light walls and a low ceiling , a great deal of light will be reflected from the walls and ceiling . This light will increase overall illumination . You may have to stop down one full stop to avoid overexposure . On the other hand , if you are shooting outdoors or in a large auditorium or gymnasium , there will be no reflection of light to help you . You may have to open up one full stop , or possibly two , to get a properly exposed picture . Always take note of your surroundings and adjust your flash exposure accordingly .
The type of reflector you use also affects the exposure and , therefore , the guide number . Published guide number tables usually specify the type of reflectors for which they were computed . Don't use the table published for a polished , high efficiency reflector if you are using one which is dull , shallow and inefficient . You may have to increase the lens opening one or more stops to get correctly exposed pictures if you use an inefficient reflector .
The third condition you should consider is the nature of your subject . For particularly dark subjects , open up a half stop ; for particularly light subjects , close down a half stop .
Finally , if the picture is important , and you want to be sure of exposing it perfectly , bracket your exposures . Make three exposures , one at what you believe to be the correct exposure , one positive color film , use only a half stop variation because of its limited exposure latitude .
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