packaged with film may also provide guide numbers
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
of the films exposure latitude.
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:
cards-one for each f/stop marked on your lens.
in the picture. Process your film normally, examine the
that best reproduces the model's skin tones. Multiply the
flash-to-subject distance) and you have the guide
number for that particular film and flash unit
you have determined the correct guide numbers for use
with various films, make up a reference chart and attach
it to your flash unit.
is governed solely by the duration of the flash.
types of artificial illumination (the same as with
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.
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.
by the flash. When this sensor is satisfied with the
flash. The closer the subject is to the lamp, the quicker
the sensor shuts off the light.
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).
Basic Photography Course