YOUR QUESTIONS ANSWERED
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Our resident Nikon expert Matthew Richards answers your questions and solves your problems. If you have a Nikon-related question, email it to mail@nphotomag.com
Q
I’ve noticed that Nikon’s various Speedlight flashguns have different power ratings or ‘Guide numbers’ but how do these work out in practice?
Gavin Yates
A
The Guide number (Gn) of a flashgun is a measure of its maximum output. The larger the number, the greater the maximum intensity of the flash. The Gn is usually specified in metres at an ISO rating of 100. It’s also generally given for the longest zoom setting of the flashgun, where the beam is at its
narrowest angle and therefore has the greatest reach.
The SB-5000’s Gn is specified with a standard illumination pattern. It shrinks with the ‘even’ illumination option and gets larger with ‘centre-weighted’ illumination.
The Nikon SB-5000 has a Gn 55 rating at ISO100, for its longest zoom length of 200mm. In practical terms, the Gn forms part of an equation where the Gn equals the distance multiplied by the aperture (f-number) for a correct exposure. So when shooting at the 200mm zoom position at full flash power, your SB-5000 would therefore provide the correct amount of illumination at 13.75m with an aperture of f/4 (13.75x4=55), shrinking to 6.875m with an aperture of f/8 (6.875x8=55).