Nikon SB-28 Guide Numbers for determining correct aperture in, To calculate the correct aperture

Guide Numbers for determining correct aperture in

Guide numbers help you determine a correct exposure or proper aperture (f/stop) when using the SB-28 in the Manual ƒ or Repeating Flash " mode.

• See page 51 for % guide numbers.

The guide number represents the amount of light (ISO 100: for m/ft) generated by the flash. With the SB-28, you can calculate a correct aperture by using the following equation and the guide number table.

To calculate the correct aperture:

Guide number (GN)

f/stop (aperture) = ——————————————

Flash shooting distance (m/ft)

To calculate the shooting distance:

Guide number (GN)

Flash shooting distance (m/ft) = ——————————

f/stop (aperture)

Guide numbers (at ISO 100: for m/ft at 20｡C/68｡F) in ƒ and " modes

Flash output			Zoom-head position
level	18mm	20mm	24mm	28mm	35mm	50mm	70mm	85mm
1/1 (full)	18/59	20/66	30/98	32/105	36/118	42/138	48/157	50/164
1/2	12.7/42	14/46	21/69	22.5/74	25.5/84	30/98	34/112	36/118
1/4	9/30	10/33	15/49	16/53	18/59	21/69	24/79	25/82
1/8	6.4/21	7/23	10.5/35	11.3/37	12.7/42	15/49	17/56	18/59
1/16	4.5/15	5/16	7.5/25	8/26	9/30	10.5/35	12/39	12.7/42
1/32	3.2/10	3.5/11	5.3/17	5.7/19	6.4/21	7.5/25	8.5/28	9/30
1/64	2.3/8	2.5/8	3.8/13	4/13	4.5/15	5.3/17	6.0/20	6.3/21

•Guide number varies with film speed.

For example, when shooting a subject located 9m (approx. 30 ft) away at 1/1 (full) flash output, with the zoom-head position at 35mm and a film speed of ISO 100, first read the guide number from the table above. In this case, it is 36 (or 118). Then divide the guide number by the shooting distance:

f/stop = 36/9 (in meters) = 4

f/stop = 118/30 (in feet) = 3.93 = approx. 4

Therefore f/4 is the correct aperture.

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