Nikon SB-50DX User Manual
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Detailed Operation
Reading Variable Aperture of Zoom Lenses
Guide Number
Some zoom lenses have variable aperture (lens’ brightness) with zooming
(variable focal length). (For details, see the lens’ instruction manual.) For
example, a lens such as the AF Zoom-Nikkor 28-105mm f/3.5-4.5D has
two maximum aperture values. Follow the instructions below to determine
aperture with such lens.
• Reading aperture in the camera’s LCD panel and/or inside the
viewfinder
• After zooming in/out for composing, set the aperture and confirm the aperture
value in the camera’s LCD panel and/or inside the viewfinder. The
compensated aperture value is displayed. Aperture value displayed in the
aperture-direct-readout scale through the camera’s viewfinder (of the Nikon F5
or other cameras that are compatible with aperture-direct-readout scale), is
fixed and not compensated as "Reading aperture with the scale on the lens"
below.
• Reading aperture with the scale on the lens
• After zooming in/out for composing, read
aperture with the aperture scale on the lens. For
the shortest focal length setting, read the
aperture value at the green index (or line index);
for the longest focal length setting, read the
value at the yellow index (or dot index). For
intermediate focal length settings, read the
aperture value between the two indexes.
• Aperture ring and aperture scale are not
provided with the G-type Nikkor lens and the
aperture cannot be read with the scale on the
lens.
• Reading aperture of the CPU lens with the Nikon F-401x/N5005,
F-401s/N4004s, or F-401/N4004 camera
• Set the camera’s exposure mode to Aperture-Priority Auto (A) or Manual (M)
and read the aperture value with the camera’s dial. Although the aperture
varies during zooming, the camera automatically compensates the lens value
for correct exposure. Setting the aperture dial to a smaller value than the lens’
smallest f-number (maximum aperture) or to a larger value than the lens’
largest f-number (minimum aperture) sets the lens aperture to the lens’
maximum or minimum aperture.
• In Programmed Auto/Auto-Multi Program (P) or Shutter-Priority Auto (S)
exposure mode, the aperture value set on the lens cannot be confirmed. In
addition, the shutter cannot be released in Manual flash operation (p. 31).
The guide number indicates the power of the flash. It varies according to
ISO film speed (sensitivity) and zoom-head position.
• Equations including guide number
• The table below shows the guide numbers for the SB-50DX with ISO 100 sensitivity:
(14mm zoom-head position: with the wide-flash adapter, p. 17).
Zoom-head position
14mm 24mm 28mm 35mm 50mm
Guide number (m/ft.)
12/39
18/59
20/66
22/72
26/85
• Use the following equation to determine the correct aperture:
f/stop =
guide number
flash-to-subject distance
Or, use the following equation to determine the flash-to-subject distance:
flash-to-subject distance =
guide number
f/stop
• For sensitivity other than ISO 100, multiply the guide number at ISO 100 by the
factors shown in the table below:
ISO film speed
25
50
200
400
800
1600
Factor
X0.5
X0.71
X1.4
X2
X2.8
X4
• For close-up flash photography, see p. 43.
• Guide number in 1/300 TTL High-Speed Sync (with the F5 camera)
• The table below shows the guide numbers for the SB-50DX with ISO 100 sensitivity:
(14mm zoom-head position: with the wide-flash adapter.)
Zoom-head position
14mm
24mm
28mm
35mm
50mm
Guide number (m/ft.)
9.5/31
10/33
11/35
12/38
14/44
• Combined guide number
• Calculating the combined guide number of two or more Speedlight units may
be necessary when performing double-flash operation (p. 54) or illuminating a
distant subject (p. 62). The combined guide number is calculated using the
following equations. The guide number varies according to ISO film speed
(sensitivity) and zoom-head position. For guide number information of the
camera’s built-in Speedlight or Accessory Speedlight other than the SB-50DX,
see your camera’s instruction manual or Speedlight’s instruction manual.
Combined guide number in double-flash operation (GN = guide number):
Combined GN =
(GN of the SB-50DX)
2
+ (GN of the camera’s built-in Speedlight)
2
Combined guide number in multiple flash operation (GN = guide number):
Combined GN = (GN of Speedlight A)
2
+ (GN of Speedlight B)
2
+ (GN of Speedlight C)
2
+ ...
SB-50DX (E) 02.12.9 2:33 PM Page 32