Nikon SB-27 instruction manual To calculate a correct aperture, To calculate the shooting distance

Guide numbers for determining correct aperture

The “guide number” represents the amount of light generated by the flash. With the SB-27 in Manual (M) Flash mode, you can calculate a correct aperture (f/stop) value by using the following equation and the “Guide number” table. —For Guide numbers in 1/300 TTL High-Speed Sync Flash

operation with the F5 camera, see page 40.

To calculate a correct aperture:

f/stop (aperture)=

guide number

flash-to-subject distance (m)

To calculate the shooting distance:

flash-to-subject distance (m)= guide number f/stop (aperture)

Guide number (at ISO 100: for m/ft)					(at 20˚C/68˚F)
	Light		Zoom-head position
		24mm	28mm	35mm	50mm
	Output
			(normal)
	1/1 (full)	25/82	27/89	30/98	34/112
	1/2	17.7/58	19/62	21.2/69	24/79
	1/4	12.5/41	13.5/44	15/49	17/56
	1/8	8.8/29	9.5/31	10.5/34	12/39
	1/16	6.2/20	6.7/22	7.4/24	8.5/28

The guide number varies with the film speed. For films other than ISO 100, multiply the above figures by the factors shown in the table below.

Adjustment factors for other ISO film speeds

ISO film speed	25	50	200	400	800	1600
Factor	⋅ 0.5	⋅ 0.71	⋅ 1.4	⋅ 2	⋅ 2.8	⋅ 4

For example, when shooting a subject under the following conditions, —Light output: 1/1 (full),

—Flash mode: Manual (M) ,

—Film speed: ISO 400,

—Zoom-head position: 35mm,

you will obtain a guide number of 60 (30 ⋅ 2) using the above table.

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