تويت
التصوير الفوتوغرافي الحديث
الجزء الأول - تصوير فلاش
الوميض الإلكتروني
ELECTRONIC FLASH
The electronic flash is also called speed light or strobe light . The electronic flash lamp operates on an entirely different principle than the ordinary flashbulb . The flashbulb emits its light when metal foil or wire is made to burn in an atmosphere of oxygen . Once the lamp is fired and the metal is burned up , the lamp is useless and must be thrown away . The electronic flash lamp does not actually burn anything . It is a gas - filled tube , something like a small neon tube , except that it is filled with a mixture of inert gases , principally zenon , and emits a white light of approximately 6000 ° K. color temperature instead of the red light emitted by neon gas . To flash the tube , a heavy current from a high voltage capacitor is passed through it for an exceedingly short period from 1/500 to 1/2000 second , or even less , depending upon the electrical circuit . The tube may be flashed repeatedly , since it has no filament , foil or wire to burn up .
The brief heavy current required to flash the tube is obtained by building up an electrical charge in a large capacitor . In portable units the power to build up the capacitor charge is obtained from batteries ; in AC operated units it is supplied by a power pack .
It takes a little time for the capacitor to become fully charged - usually from three to ten seconds . That is why you must wait for this period of time between exposures . However , this short charging time is seldom any handicap . Figure 8 shows a small battery - operated electronic flash unit of the type which is very popular for use on 35mm cameras where extreme portability is important . Figure 9 shows a somewhat larger unit more suitable for the press and commercial photographer because of its higher output . The power supply in such units is usually carried separately as shown . In Figure 10 , we show some large studio type electronic flash equipment .
CHARACTERISTICS OF ELECTRONIC FLASH . Except for the matter of synchronization which has been discussed earlier in this Assignment , the electronic flash is used in much the same way as the more common flashbulb . It is , of course , unnecessary to change bulbs after every exposure , so you don't have to carry a pocket full of bulbs . On the other hand , the electronic unit is larger and heavier for an equal light output , not to mention its much greater cost . The electronic flash unit is not intended to replace the flashbulb . Both have their uses . As an example , the lightweight portable electronic flash pictured in Figure 8 , does not give as much light as the flashbulb , so its light does not carry so far . On the other hand , the electronic flash with its speed is better for very fast action pictures such as the one in Figure 11 .
Figure 8 Extremely lightweight but highly efficient electronic flash unit .
Figure 9 An electronic flash with high light output for press and commercial work .
Figure 10 Studio type electronic flash shown with an " umbrella " diffuser
Figure 11 ( Below ) Electronic flash is especially useful for sports pictures such as this ..
The electronic flash is so fast that the eyes do not have time to react to it . Your subject can look right into the flash when it goes off without being disturbed by it .
Some high power commercial electronic flash units have a flash duration of less than 1/2000 second . Such very short exposures tend to produce soft negatives . If you need crisp , snappy negatives , it will be necessary to increase film development time . The amount of increase in development time depends upon the duration of the flash , the type of film and the developer . As a suggestion , try a 30 % increase to start with and modify it as necessary to give you proper negative contrast . The small portable electronic flash units generally flash at 1/1000 second , and at this speed no special film processing techniques are required .
DETERMINING EXPOSURE FOR ELECTRONIC FLASH . Exposure with electronic flash is determined in the same way as with ordinary flash - by the use of exposure guide numbers . A table of guide numbers is always given in the operating instructions accompanying the equipment . As in the case of the ordinary flashbulb , it is advisable to watch your exposure carefully at first and modify the guide numbers if you find it necessary .
It is not possible to give exposure guide numbers here as we did for ordinary . flashbulbs . The light output of an ordinary flashbulb is fixed and does not change no matter how you fire it . The light output of an electronic flash unit is determined not only by the lamp itself , but by the amount of energy fed into it by the power unit with which it is supplied . This energy is measured in watt - seconds . You must , therefore , follow the manufacturer's instructions . Your will note , however , that guide numbers for electronic flash do not take shutter speed into account . This is because the flash is always faster than the shutter , so the flash determines the exposure time .
Most portable electronic flash units are equipped with simple calculators so that merely setting a small dial to the film speed enables the user to read off the correct lens opening for any subject distance . This eliminates all exposure calculations .
ELECTRONIC FLASH WITH FOCAL PLANE SHUTTERS . As we have mentioned earlier in this Assignment , fast exposures with a focal plane shutter are produced by allowing a slit in the shutter to travel across the face of the film . This makes it impossible to use fast shutter speeds with electronic flash . Generally speaking , the focal plane shutter should be set from 1/30 second to 1/60 second when electronic flash is used with focal plane shutters . At this slow shutter speed , the focal plane shutter is opened to the full width of the picture area during exposure . Actual time of exposure , of course , is determined by the duration of the flash which is usually 1/1000 second . In most 35mm cameras , the shutter speed recommended for electronic flash appears in red on the shutter speed dial . Longer exposures may be used , if necessary , for other reasons , but shorter speeds will result in blank areas on the film .
ADVANTAGES OF ELECTRONIC FLASH . Electronic flash has a number of important advantages over ordinary flash to compensate for its greater cost and its usually more limited light output . These are :
1. It stops all action and freezes normal camera movement , thus producing very sharp pictures .
2. Synchronization is constant and reliable . Since the flash is so quick that it determines the exposure time , only one guide number is used for all shutter speeds
3. Tube replacement is no problem - one tube should last for at least 10,000 flashes .
4. The soft light it produces is excellent for portraiture of children ; its short duration does not frighten very young children .
5. The flashes are of constant duration , intensity and color , making them ideal for color
6. Operating cost is low . If it is used a great deal , its low operating cost will overcome its higher initial cost .
الجزء الأول - تصوير فلاش
الوميض الإلكتروني
ELECTRONIC FLASH
The electronic flash is also called speed light or strobe light . The electronic flash lamp operates on an entirely different principle than the ordinary flashbulb . The flashbulb emits its light when metal foil or wire is made to burn in an atmosphere of oxygen . Once the lamp is fired and the metal is burned up , the lamp is useless and must be thrown away . The electronic flash lamp does not actually burn anything . It is a gas - filled tube , something like a small neon tube , except that it is filled with a mixture of inert gases , principally zenon , and emits a white light of approximately 6000 ° K. color temperature instead of the red light emitted by neon gas . To flash the tube , a heavy current from a high voltage capacitor is passed through it for an exceedingly short period from 1/500 to 1/2000 second , or even less , depending upon the electrical circuit . The tube may be flashed repeatedly , since it has no filament , foil or wire to burn up .
The brief heavy current required to flash the tube is obtained by building up an electrical charge in a large capacitor . In portable units the power to build up the capacitor charge is obtained from batteries ; in AC operated units it is supplied by a power pack .
It takes a little time for the capacitor to become fully charged - usually from three to ten seconds . That is why you must wait for this period of time between exposures . However , this short charging time is seldom any handicap . Figure 8 shows a small battery - operated electronic flash unit of the type which is very popular for use on 35mm cameras where extreme portability is important . Figure 9 shows a somewhat larger unit more suitable for the press and commercial photographer because of its higher output . The power supply in such units is usually carried separately as shown . In Figure 10 , we show some large studio type electronic flash equipment .
CHARACTERISTICS OF ELECTRONIC FLASH . Except for the matter of synchronization which has been discussed earlier in this Assignment , the electronic flash is used in much the same way as the more common flashbulb . It is , of course , unnecessary to change bulbs after every exposure , so you don't have to carry a pocket full of bulbs . On the other hand , the electronic unit is larger and heavier for an equal light output , not to mention its much greater cost . The electronic flash unit is not intended to replace the flashbulb . Both have their uses . As an example , the lightweight portable electronic flash pictured in Figure 8 , does not give as much light as the flashbulb , so its light does not carry so far . On the other hand , the electronic flash with its speed is better for very fast action pictures such as the one in Figure 11 .
Figure 8 Extremely lightweight but highly efficient electronic flash unit .
Figure 9 An electronic flash with high light output for press and commercial work .
Figure 10 Studio type electronic flash shown with an " umbrella " diffuser
Figure 11 ( Below ) Electronic flash is especially useful for sports pictures such as this ..
The electronic flash is so fast that the eyes do not have time to react to it . Your subject can look right into the flash when it goes off without being disturbed by it .
Some high power commercial electronic flash units have a flash duration of less than 1/2000 second . Such very short exposures tend to produce soft negatives . If you need crisp , snappy negatives , it will be necessary to increase film development time . The amount of increase in development time depends upon the duration of the flash , the type of film and the developer . As a suggestion , try a 30 % increase to start with and modify it as necessary to give you proper negative contrast . The small portable electronic flash units generally flash at 1/1000 second , and at this speed no special film processing techniques are required .
DETERMINING EXPOSURE FOR ELECTRONIC FLASH . Exposure with electronic flash is determined in the same way as with ordinary flash - by the use of exposure guide numbers . A table of guide numbers is always given in the operating instructions accompanying the equipment . As in the case of the ordinary flashbulb , it is advisable to watch your exposure carefully at first and modify the guide numbers if you find it necessary .
It is not possible to give exposure guide numbers here as we did for ordinary . flashbulbs . The light output of an ordinary flashbulb is fixed and does not change no matter how you fire it . The light output of an electronic flash unit is determined not only by the lamp itself , but by the amount of energy fed into it by the power unit with which it is supplied . This energy is measured in watt - seconds . You must , therefore , follow the manufacturer's instructions . Your will note , however , that guide numbers for electronic flash do not take shutter speed into account . This is because the flash is always faster than the shutter , so the flash determines the exposure time .
Most portable electronic flash units are equipped with simple calculators so that merely setting a small dial to the film speed enables the user to read off the correct lens opening for any subject distance . This eliminates all exposure calculations .
ELECTRONIC FLASH WITH FOCAL PLANE SHUTTERS . As we have mentioned earlier in this Assignment , fast exposures with a focal plane shutter are produced by allowing a slit in the shutter to travel across the face of the film . This makes it impossible to use fast shutter speeds with electronic flash . Generally speaking , the focal plane shutter should be set from 1/30 second to 1/60 second when electronic flash is used with focal plane shutters . At this slow shutter speed , the focal plane shutter is opened to the full width of the picture area during exposure . Actual time of exposure , of course , is determined by the duration of the flash which is usually 1/1000 second . In most 35mm cameras , the shutter speed recommended for electronic flash appears in red on the shutter speed dial . Longer exposures may be used , if necessary , for other reasons , but shorter speeds will result in blank areas on the film .
ADVANTAGES OF ELECTRONIC FLASH . Electronic flash has a number of important advantages over ordinary flash to compensate for its greater cost and its usually more limited light output . These are :
1. It stops all action and freezes normal camera movement , thus producing very sharp pictures .
2. Synchronization is constant and reliable . Since the flash is so quick that it determines the exposure time , only one guide number is used for all shutter speeds
3. Tube replacement is no problem - one tube should last for at least 10,000 flashes .
4. The soft light it produces is excellent for portraiture of children ; its short duration does not frighten very young children .
5. The flashes are of constant duration , intensity and color , making them ideal for color
6. Operating cost is low . If it is used a great deal , its low operating cost will overcome its higher initial cost .
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