>The guide number of a flash is a rating of the power output >measured in meters or feet. The number is handy when trying >to compare different flashes when purchasing of one.
Unfortunately, though, getting the Guide Numbers in the same form can be tricky. Some brands quote GN for the maximum zoom of the flash, or for higher ISO both of which tend to increase the GN and exaggerate the power of the flash.
>After the purchase, the guide number is not that useful since >the camera and flash displays will assist you in getting the >correct exposure for your picture without you knowing or >caring what the guide number is.
>Just for comparison: >d200 Guide Number of 12/39 (at ISO 100, m/ft) >sb910 Guide number of 34/111.5 (at ISO 100, m/ft) >This means the the sb910 flash will shoot about 3x further.
Of course, the D200 has a fixed flash illumination pattern, so the GN is fixed. The SB910 has different illumination patterns, DX or FX mode, and variable zoom settings. That gives a range of Guide Numbers. The SB900 has Guide Numbers of 13 to 58, all at ISO 100, in metres; I suspect the 910 is similar. See what I was saying about having the Guide Number in the same form?