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.
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).
at f/2.8, is 1.6 to 50 feet.

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