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Flash – The Comprehensive Camera Flash Buying Guide


Power will be the main consideration you should be concentrating on when it comes to flash specification. The power of the flash is rated by its GUIDE NUMBER or GN value.

Without going too technical (not the purpose of this article), you should always choose a flash with the highest GN that you can afford. The higher the GN, the more light the flash can emit. You can always reduce flash output but you cannot increase output beyond what the flash is capable of producing.

The basic definition of guide number is the equation GN= flash distance to subject x aperture value. If you want to learn more about GN, head to Wikipedia and type “Guide Number”.

Sounds simple, right? Actually, it is!

Here’s the warning though, some manufacturers overstate their GN or skips some crucial parameters when they indicate their product’s GN.

Assuming the flash is producing x-amount of light, the intensity of the light (GN) is affected by how much of that light output is spread out. The more WIDTH the light can cover, the less intense the light will appear. Conversely, the narrower the beam, the brighter the light will be.

Confused? Think of adjustable beam flashlights (like Mag Light flash lights) for a second. The bulb’s light output remains constant as soon as you turn on the flashlight, but if you twist the light’s beam narrower, it appears brighter as the beam is more concentrated on a particular spot. When you turn it the other way and spread the light across a wider area, the light will not be able to reach as far and will appear less intense.

It’s the same with camera flashguns as most flash units have a zoom “head” that switches from wide-angle to telephoto so that the beam of light emitted matches the focal length of your camera lens’ coverage.

Most flash manufacturers rate their GN in meters with the assumption of the film/sensor’s sensitivity at ISO 100 (lowest base setting) using the maximum zoom (narrowest beam) of the flash head.

For example:

  • Canon 580EX Mark II flash is rated at GN 58 @ ISO 100, 105mm (hence the model 580EX, the lower model 430EX has a GN of 43).
  • Nikon SB900 is rated at GN 49.5 @ ISO 100, 105mm. On paper, the SB900 throws out less light (it does, but not that much less) than the 580EX II but the SB900 can zoom to 200mm (compared to 105mm) which makes the comparison quite even.
  • Pentax AF540-FGZ is rated at GN 54 at ISO 100, 85mm. On the opposite end, Pentax’s flagship flash can only zoom out to 85mm, if it has a 105mm option, the power will be quite similar to what Canon and Nikon offers as well (GN 56-58 range).

For the name-brand flashes, the top-of-the-line offerings are quite similar and predictable.

Returning to the “warning” statement, some manufacturers bank on folks not studying the ratings more carefully, using a higher ISO or a higher zoom figure to rate their flashes. A flash’s GN will increase if the rated ISO was set to a higher value (ISO 200, 400 etc.) or has when charts compare a flash zoomed at 200mm vs an OEM flash at 85mm, for example.

Just make sure you’re comparing near-identical configuration when checking spec sheets and research online. Some flash units may be rated correctly on paper, but requires special power sources to reach that rated amount or can only sustain that amount of light for a few shots.

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