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Mastering the Nikon D300/D300S by Darrell Young

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What Is a Guide Number?

The guide number (GN) for a flash unit measures how well it can light a subject at a specific ISO sensitivity and with a precise angle of view (wide-angle vs. telephoto lens). To put it simply, a higher guide number means the flash is more powerful, all other things being equal.

Be careful when you are deciding on an external flash unit to use, whether it is a genuine Nikon Speedlight or an aftermarket unit. Simply comparing the GN is not enough. You must understand the settings the GN is based upon. Many flash units have zoom capability and will be able to light up subjects farther away when zoomed out. However, imagine buying a flash unit from a manufacturer who publishes the GN based on a longer zoom position and then ...

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