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التصوير الفوتوغرافي الحديث
الجزء الأول - تصوير فلاش
تزامن الفلاش
FLASH SYNCHRONIZATION
The light from a flash lamp is extremely brilliant - at its peak it is greater than average sunlight on earth . However , its peak is of short duration . That is why the shutter must open to expose the film at the exact instant the flash reaches maximum intensity . This is done by synchronizing the lamp and the shutter .
We have stated that it takes from 15 to 20 milliseconds for the flashbulb to reach peak brilliance . It also takes an appreciable length of time for a between - the unfortunately , these two intervals are not equal . The average between - the - lens shutter takes from 2 to 4 milliseconds to open fully and requires about the same time to close .
Since the shutter is only partly open during the opening and closing portions of the exposure cycle , the average shutter requires a total elapse time of 12 to 14 milliseconds to produce an effective exposure of 10 milliseconds ( 1/100 second ) . Thus , for fast exposure of 1/100 second or less - if the shutter release and flash lamp switch are pressed at the same time - the shutter will have opened and closed at about the time the flash lamp begins to fire . The solution is to delay the shutter opening an accurately determined length of time .
TYPE M SYNCHRONIZATION . With this type of synchronization the electrical contact is made first , then the opening of the shutter follows by about 17 % milliseconds . The shutter itself takes an average of 2½ milliseconds to open , so it is fully open 20 milliseconds after the button is pressed . This , you will recall , is the time required for an M type flash lamp to reach peak output .
The delay mechanism can be mechanical ( gear train , springs , etc. inside the shutter ) or electrical ( solenoid attached to the exposure release outside the shutter ) . External synchronizers should be adjustable , since shutters differ in their delay time .
In Figure 2 , we show what happens with correct synchronization for a shutter speed of 1/200 second . The shaded curve represents the light output of the lamp , the wedgeshaped outline represents the opening and closing of the shutter . Note that the shutter is open during the peak of light output . In Figure 3 the shutter is out of synchronization and opened too soon . Here light is entering the camera for less than half the exposure time - the result is a badly underexposed picture .
Accurate synchronization is essential at the faster shutter speeds , but it may not be too important for slow shutter speeds . Figure 4 shows what happens for a 12½ millisecond delay instead of the correct 17 % with a shutter speed of 1/50 second ( 20 milliseconds ) . The shutter is open long enough to capture the full output of the lamp in spite of the incorrect synchronization .
Let us look at one more example . Suppose we have no means of delaying shutter opening - can we still take flash picture ? Yes , we can and this is shown in Figure 5. Here the exposure is 1/25 second ( 40 milliseconds ) . Note that even though the shutter is fully open in 3 milliseconds , long before the flashbulb reaches peak lumens , it takes in the full light output of the flash lamp .
Many simple cameras have this type of synchronization . The shutter while not actually synchronized , has a set of electrical contacts , which make contact when the shutter is pressed , and the user is told to take all flash pictures at 1/25 second .
Figure 4 Accurate synchronization is not so important when slow shutter speeds are used .
Figure When the shutter has no time delay at all , it is still possible to take flash pictures by using very slow shutter speeds ( 1/25 or 1/30 second ) .
TYPE X SYNCHRONIZATION . Electronic flash lamps flash to full brilliance in less than one millisecond . To make them practical , therefore , the starting time of the flash rather than the shutter must be delayed . For that reason , shutters with electronic or X synchronization are designed so that the electrical contact is made when the shutter is fully open . In that way , none of the flash illumination is lost . The actual shutter speed is not at all important because the flash itself is so short
that it determines the exposure time . Electronic flash duration is usually 1/1000 second for portable flash units and 1/2000 second to 1/5000 second for large studio type flash equipment .
Some electronic flash units have electrical circuits that may be adjusted to delay the firing of the lamp for a specified number of milliseconds , so they can also be used with M synchronized shutters . This feature also permits the electronic flash units to be used together with an ordinary flash lamp if more than one light source is required . Without the electrical delay circuit , the two lamps would flash at different times , giving a double image if the subject is not stationary .
FP SYNCHRONIZATION . A focal plane shutter has a narrow slit which travels across the face of the film . The width of the slit and the spring tension actuating the shutter may be set so each point on the film is exposed for , let us say , 1/250 second as the slit goes by , but the slit may take 1/50 second or longer to traverse the whole film area . This requires a different type of lamp , as was explained earlier , but the shutter delay is about the same as for M synchronization . If an FP type flashbulb is not available , an M type bulb may be used at exposures of 1/30 second , as in the case of unsynchronized between - the - lens shutters . However , some focal plane shutters permit faster speeds with some of the M type flash bulbs - up to 1/125 second in some cases . Be sure to check the instruction book of your camera to see what shutter speeds are possible with it .
الجزء الأول - تصوير فلاش
تزامن الفلاش
FLASH SYNCHRONIZATION
The light from a flash lamp is extremely brilliant - at its peak it is greater than average sunlight on earth . However , its peak is of short duration . That is why the shutter must open to expose the film at the exact instant the flash reaches maximum intensity . This is done by synchronizing the lamp and the shutter .
We have stated that it takes from 15 to 20 milliseconds for the flashbulb to reach peak brilliance . It also takes an appreciable length of time for a between - the unfortunately , these two intervals are not equal . The average between - the - lens shutter takes from 2 to 4 milliseconds to open fully and requires about the same time to close .
Since the shutter is only partly open during the opening and closing portions of the exposure cycle , the average shutter requires a total elapse time of 12 to 14 milliseconds to produce an effective exposure of 10 milliseconds ( 1/100 second ) . Thus , for fast exposure of 1/100 second or less - if the shutter release and flash lamp switch are pressed at the same time - the shutter will have opened and closed at about the time the flash lamp begins to fire . The solution is to delay the shutter opening an accurately determined length of time .
TYPE M SYNCHRONIZATION . With this type of synchronization the electrical contact is made first , then the opening of the shutter follows by about 17 % milliseconds . The shutter itself takes an average of 2½ milliseconds to open , so it is fully open 20 milliseconds after the button is pressed . This , you will recall , is the time required for an M type flash lamp to reach peak output .
The delay mechanism can be mechanical ( gear train , springs , etc. inside the shutter ) or electrical ( solenoid attached to the exposure release outside the shutter ) . External synchronizers should be adjustable , since shutters differ in their delay time .
In Figure 2 , we show what happens with correct synchronization for a shutter speed of 1/200 second . The shaded curve represents the light output of the lamp , the wedgeshaped outline represents the opening and closing of the shutter . Note that the shutter is open during the peak of light output . In Figure 3 the shutter is out of synchronization and opened too soon . Here light is entering the camera for less than half the exposure time - the result is a badly underexposed picture .
Accurate synchronization is essential at the faster shutter speeds , but it may not be too important for slow shutter speeds . Figure 4 shows what happens for a 12½ millisecond delay instead of the correct 17 % with a shutter speed of 1/50 second ( 20 milliseconds ) . The shutter is open long enough to capture the full output of the lamp in spite of the incorrect synchronization .
Let us look at one more example . Suppose we have no means of delaying shutter opening - can we still take flash picture ? Yes , we can and this is shown in Figure 5. Here the exposure is 1/25 second ( 40 milliseconds ) . Note that even though the shutter is fully open in 3 milliseconds , long before the flashbulb reaches peak lumens , it takes in the full light output of the flash lamp .
Many simple cameras have this type of synchronization . The shutter while not actually synchronized , has a set of electrical contacts , which make contact when the shutter is pressed , and the user is told to take all flash pictures at 1/25 second .
Figure 4 Accurate synchronization is not so important when slow shutter speeds are used .
Figure When the shutter has no time delay at all , it is still possible to take flash pictures by using very slow shutter speeds ( 1/25 or 1/30 second ) .
TYPE X SYNCHRONIZATION . Electronic flash lamps flash to full brilliance in less than one millisecond . To make them practical , therefore , the starting time of the flash rather than the shutter must be delayed . For that reason , shutters with electronic or X synchronization are designed so that the electrical contact is made when the shutter is fully open . In that way , none of the flash illumination is lost . The actual shutter speed is not at all important because the flash itself is so short
that it determines the exposure time . Electronic flash duration is usually 1/1000 second for portable flash units and 1/2000 second to 1/5000 second for large studio type flash equipment .
Some electronic flash units have electrical circuits that may be adjusted to delay the firing of the lamp for a specified number of milliseconds , so they can also be used with M synchronized shutters . This feature also permits the electronic flash units to be used together with an ordinary flash lamp if more than one light source is required . Without the electrical delay circuit , the two lamps would flash at different times , giving a double image if the subject is not stationary .
FP SYNCHRONIZATION . A focal plane shutter has a narrow slit which travels across the face of the film . The width of the slit and the spring tension actuating the shutter may be set so each point on the film is exposed for , let us say , 1/250 second as the slit goes by , but the slit may take 1/50 second or longer to traverse the whole film area . This requires a different type of lamp , as was explained earlier , but the shutter delay is about the same as for M synchronization . If an FP type flashbulb is not available , an M type bulb may be used at exposures of 1/30 second , as in the case of unsynchronized between - the - lens shutters . However , some focal plane shutters permit faster speeds with some of the M type flash bulbs - up to 1/125 second in some cases . Be sure to check the instruction book of your camera to see what shutter speeds are possible with it .
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