Univex SB-900 D–23, Calculating the correct aperture, Obtaining the correct flash output level

in the Distance-priority, Manual and Repeating flash modes

■Calculating the correct aperture

Determine the guide number by referring to the guide number table. The correct guide number depends on the flash output level and zoom position. Next, calculate the correct aperture by using the equation below. The correct aperture depends on ISO sensitivity and shooting distance (m/ft.).

f/stop (aperture) = Guide number (GN at ISO 100; m/ft.) × ISO sensitivity factor ÷ Shooting distance (m/ft)

For example, for an ISO sensitivity of 400 (or ISO sensitivity factor: 2), with the zoom position adjusted to 28 mm, and the flash output level set to M1/1:

Referring to the guide number table, determine the guide number as 30/98.4 (m/ft.). When the shooting distance is 4 m (13.1 ft.), the correct aperture value is:

30/98.4 (GN at ISO 100, m/ft) × 2 (ISO sensitivity factor for ISO 400) ÷ 4/13.1 (shooting distance; m/ft.) = 15 (aperture)

•Set the smaller aperture value (larger f-number) than calculated above on the camera.

■Obtaining the correct flash output level

When the shooting distance and aperture are fixed, calculate the guide number (GN at ISO 100; m/ft.) by using the equation below, based on shooting distance and aperture:

Guide number (GN at ISO 100; m/ft.) = Shooting distance (m/ft) × Aperture (f/) ÷ ISO sensitivity factor

Referring to the guide number table, determine an appropriate flash output level corresponding to the guide number obtained above, then set the same value on the SB-900. For example, for the shooting distance of 6 m (19.7 ft) with an aperture of f/8 and the ISO sensitivity of 400 (ISO sensitivity factor: 2):

6/19.7 (shooting distance; m/ft.) × 8 (aperture value) ÷ 2 (ISO sensitivity factor of ISO 400) = 24/79 (GN at ISO 100; m/ft.)

•When the zoom position is adjusted to 35 mm, set M1/2 flash output level on the SB-900, which can be obtained from GN 24/79 (m/ft.) corresponding to the zoom position of 35 mm referring to the guide number table.

•In Distance-priority manual flash operation, the correct flash output level is automatically determined by the SB-900 according to the shooting distance, aperture and ISO sensitivity that are set.

■Calculating the shooting distance

Determine the guide number by referring to the guide number table. The correct guide number depends on the flash output level and zoom position. Next, calculate the shooting distance by using the equation below. The shooting distance (m/ft.) depends on ISO sensitivity and aperture.

Shooting distance (m/ft.) = Guide number (GN at ISO 100; m/ft.) × ISO sensitivity factor ÷ Aperture (f/)

D

Flash modes and functions

D–23