Nikon 50DX Instruction Manual - Page 17

Reading Variable Aperture of Zoom Lenses 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). 32 Guide Number 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 Factor 25 50 200 400 800 1600 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 + ... 33 Detailed Operation