Basic Photography Course

provided with each electronic flash unit. Information
packaged with film may also provide guide numbers

appropriate to their speed in regard to the various powers

of electronic flash units. Manufacturers tend to overrate
the power of their electronic flash units. When guide
numbers are assigned by the manufacturer, they base the
guide number on an average reflective subject and in a
room with 10-foot light-coloredceilings. By using these

methods, the manufacturers are able to take advantage
of the films exposure latitude.

Like exposure meters, guide numbers are not

infallible and some variation from assigned values
should be expected. To ensure accuracy of the flash unit,
you must check the efficiency of your electronic flash
unit to determine your own reliable guide numbers. The
steps used to check efficiency are as follows:

1. Place your flash unit (on the camera) exactly 10

feet from a live model who is holding a series of
cards-one for each f/stop marked on your lens.

2. Load the camera with the type of film you want

to test.

3. Focus the camera on the model and make an

exposure at each of the f/stops marked on the cards.

For each exposure, instruct the model to hold up the card

marked with the f/stop to be used so it shows noticeably
in the picture. Process your film normally, examine the

proof sheet or slides carefully, and choose the one shot
that best reproduces the model's skin tones. Multiply the

f/stop on the card in that picture by 10 (the
flash-to-subject distance) and you have the guide
number for that particular film and flash unit

combination. If, for example, the best exposure was

made at f/8, the guide number is 80 (8 x 10 = 80). Once
you have determined the correct guide numbers for use
with various films, make up a reference chart and attach
it to your flash unit.

Correct exposure with electronic flash depends

upon four factors:

The power or light output of the flash unit

The IS0 speed of the film being used

The flash-to-subject distance

The f/stop setting

Shutter speed is not a factor since the time of exposure
is governed solely by the duration of the flash.

Notice we always speak of flush-to-subject

distance, never camera-to-subject distance. With all
types of artificial illumination (the same as with

sunlight), the only consideration is the amount of light
reflected from the subject. The distance between the
camera and the subject has no bearing on exposure.
When the flash is used off of the camera, the basic f/stop
is still calculated with the flash-to-subject distance.

AUTOMATIC ELECTRONIC
FLASH UNITS

Most electronic flash units can be operated in an

automatic exposure mode. An automatic flash unit
eliminates the need to determine the correct f/stop for
each flash-to-subject distance, providing the subject is
within the flash distance range of the unit.

On the front of an automatic flash unit, a sensor

reads the light reflected from the subject that is produced
by the flash. When this sensor is satisfied with the

amount of light received, it automatically shuts off the
flash. The closer the subject is to the lamp, the quicker
the sensor shuts off the light.

Some automatic electronic flash units allow you to

select two or more apertures to control depth of field. To
determine an f/stop in the automatic mode, you can use
the calculator dial, located on the unit that is being used.
By matching the indicator to an IS0 film speed number
on the dial, you can use the f/stop within a minimum and
maximum distance. Once an f/stop is selected and set,
it becomes a constant factor regardless of the flash-
to-subject distance, providing it is within the flash
distance range of the unit. This feature allows the
photographer to move closer to or farther away from a
subject without having to calculate an f/stop for each
change of flash-to-subject distance (fig. 5-25).

Figure 5-25.The automatic operating range, using JSO 100 film

at f/2.8, is 1.6 to 50 feet.

5-31

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Basic Photography Course

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> provided with each electronic flash unit. Information packaged with film may also provide guide numbers appropriate to their speed in regard to the various powers of electronic flash units. Manufacturers tend to overrate the power of their electronic flash units. When guide numbers are assigned by the manufacturer, they base the guide number on an average reflective subject and in a room with 10-foot light-coloredceilings. By using these methods, the manufacturers are able to take advantage of the films exposure latitude. Like exposure meters, guide numbers are not infallible and some variation from assigned values should be expected. To ensure accuracy of the flash unit, you must check the efficiency of your electronic flash unit to determine your own reliable guide numbers. The steps used to check efficiency are as follows: 1. Place your flash unit (on the camera) exactly 10 feet from a live model who is holding a series of cards-one for each f/stop marked on your lens. 2. Load the camera with the type of film you want to test. 3. Focus the camera on the model and make an exposure at each of the f/stops marked on the cards. For each exposure, instruct the model to hold up the card marked with the f/stop to be used so it shows noticeably in the picture. Process your film normally, examine the proof sheet or slides carefully, and choose the one shot that best reproduces the model's skin tones. Multiply the f/stop on the card in that picture by 10 (the flash-to-subject distance) and you have the guide number for that particular film and flash unit combination. If, for example, the best exposure was made at f/8, the guide number is 80 (8 x 10 = 80). Once you have determined the correct guide numbers for use with various films, make up a reference chart and attach it to your flash unit. Correct exposure with electronic flash depends upon four factors: The power or light output of the flash unit The IS0 speed of the film being used The flash-to-subject distance The f/stop setting Shutter speed is not a factor since the time of exposure is governed solely by the duration of the flash. Notice we always speak of flush-to-subject distance, never camera-to-subject distance. With all types of artificial illumination (the same as with sunlight), the only consideration is the amount of light reflected from the subject. The distance between the camera and the subject has no bearing on exposure. When the flash is used off of the camera, the basic f/stop is still calculated with the flash-to-subject distance. AUTOMATIC ELECTRONIC FLASH UNITS Most electronic flash units can be operated in an automatic exposure mode. An automatic flash unit eliminates the need to determine the correct f/stop for each flash-to-subject distance, providing the subject is within the flash distance range of the unit. On the front of an automatic flash unit, a sensor reads the light reflected from the subject that is produced by the flash. When this sensor is satisfied with the amount of light received, it automatically shuts off the flash. The closer the subject is to the lamp, the quicker the sensor shuts off the light. Some automatic electronic flash units allow you to select two or more apertures to control depth of field. To determine an f/stop in the automatic mode, you can use the calculator dial, located on the unit that is being used. By matching the indicator to an IS0 film speed number on the dial, you can use the f/stop within a minimum and maximum distance. Once an f/stop is selected and set, it becomes a constant factor regardless of the flash- to-subject distance, providing it is within the flash distance range of the unit. This feature allows the photographer to move closer to or farther away from a subject without having to calculate an f/stop for each change of flash-to-subject distance (fig. 5-25). Figure 5-25.The automatic operating range, using JSO 100 film at f/2.8, is 1.6 to 50 feet. 5-31 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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