Guide Number In Fp High-Speed Sync Flash - Nikon SB-26 Instruction Manual

11 Confirm subject is at the distance shown by the
indicator bar in the SB-26's LCD panel (from
step 10).
To determine the subject-to-camera distance, read the dis-
tance scale on the lens barrel.
If the bar location matches the subject-to-camera distance
from steps 10 and 11, the subject will be correctly exposed
with the selected aperture, shutter speed, and light output
amount.
If the subject-to-camera distance does not match the appro-
priate shooting distance, readjust the indicator bars by:
a. Pressing button to switch from FP1 ( with )
to FP2, or vice versa. The bar location then
moves to show an alternate shooting distance.
b. Moving closer to or further from the subject.
c. Choose a different number for the zoom-head posi-
tion (altering guide number).
12 Look into camera's viewfinder again, then compose
and lightly press the shutter release button to check
that the ready-light is on in the camera's viewfinder .
Ready-light also appears on the SB-26 .
13 Fully depress shutter release button to fire flash.
102

Guide number in FP High-Speed Sync Flash

The "guide number" helps you determine an exact flash-
shooting (flash-to-subject) distance for the selected aper-
ture (f/stop number). FP guide numbers vary with the ISO
film speed in use, shutter speed and zoom-head position.
Use the equation below for your calculations, and check
the following chart for guide numbers. This same equation
can be applied to determine an appropriate aperture once
the distance is known.
[guide number]
[flash-shooting distance] =
[f/stop]
For example, in FP1 operation with an aperture of f/4, a
shutter speed of 1/500 sec., a zoom-head position of 35mm
and a film speed of ISO 100, the chart recommends a guide
number of 12 (or 39 for feet):
If measuring in meters;
12
[flash-shooting distance] = = 3
4
if measuring in feet;
39
[flash-shooting distance] = = 9.75
4
Next, adjust subject and flash/camera location for 3 meters,
or 9.75 feet, to obtain correct exposure in FP1 flash opera-
tion.
FP1 guide number (at ISO 100; for meters/feet)
Zoom-head position
Shutter
speed
24mm 28mm 35mm 50mm 70mm 85mm
1/250 14/46 15/50 17/56 20/65 23/74 24/77
1/500 10/33 11/36 12/39 14/46 16/52 17/56
1/1000 7/23 7.5/25 8.5/28 10/33 11/36 12/39
1/2000 5/16 5.3/17 6/20 7/23 8/26 8.5/28
1/4000 3.5/11 3.7/12 4.2/14 5/16 5.6/18 6/20
FP2 guide number (at ISO 100; for meters/feet)
Zoom-head position
Shutter
speed
24mm 28mm 35mm 50mm 70mm 85mm
1/250 10/33 11/36 12/39 14/46 16/52 17/56
1/500 7/23 7.5/25 8.5/28 10/33 11/36 12/39
1/1000 5/16 5.3/17 6/20 7/23 8/26 8.5/28
1/2000 3.5/11 3.7/12 4.2/14 5/16 5.6/18 6/20
1/4000 2.5/8.2 2.6/8.5 3/10 3.5/11 4/13 4.2/14
Chapter
FP High-Speed Sync Flash
Using a film speed other than ISO 100
For film speeds other than ISO 100, multiply the above fig-
ures by the factors shown in the following chart.
If the film speed in the previous example had been ISO
400 rather than ISO 100:
12
[flash-shooting distance] = x 2 = 3 x 2 = 6
4
if measuring in feet;
39
[flash-shooting distance] = x 2 = 9.75 x 2 =19.5
4
You should have obtained 6 meters, or 19.5 feet, for correct
exposure.
Adjustment factors for other ISO film speeds
ISO film speed 25 50 100 200 400 800 1600
Factor x 0.5 x 0.7 x 1 x 1.4 x 2 x 2.8 x 4
103
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