While working on the fill flash assignment I realized I could use a second speedlight. I shoot Nikon, and I have been using a SB-600 with Ni-MH batteries for about 2 years. I am probably going make the SB-900 my next purchase along with a 9’ coiled extension cable. Both units have similar features and operations, such as auto TTL exposure, FP or high speed sync, remote trigger via IR beam, and use in multi-unit lighting. As you would expect the SB-900 is more powerful than the 600. The guide number comparison is below for ISO 200 and 35mm zoom position.
Guide numbers indicate power and are also used in manually calculating exposure.
Many shooters ignore manual flash operation but it can give you more control of the lighting. To use a Guide Number (GN), first choose an ISO value and measure in either feet or meters. Then calculate exposure by manipulating this formula
GN = subject distance X Aperture number (f) 48 = 8.57m X f 5.6
The basic relationship between subject distance and Aperture
GN / Aperture number (f) = subject distance 48 / f 5.6 = 8.57m
A flash of GN 48, shot at f 5.6 would yield a good exposure at 8.57m distance
GN / subject distance = Aperture number (f)
A flash of GN 48, shot at a subject distance of 8.57m would yield a good exposure at f 5.6
Alternatively, you could test shoot and inspect the histogram and/or the preview until you got the desired result.