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All flash units are given a rating, called a guide number, that is simply a number that is a guide to how powerful the flash is. Now, as you might expect, a higher guide number indicates a flash with more power. A flash with more power, though, is also going to be physically larger than a flash with less power. So if you want to keep the size and weight of your bag down then you may not want to buy a flash with more power than you think you'll need. Using the guide number, you can quickly get a sense for what a flash can do lighting-wise.
Guide numbers are specific to a particular focal length and aperture, and your flash's manual should list the guide numbers for different focal lengths. For example, according to the manual, this Canon 580EX has a guide number, or range, of 190 feet when shooting with a 105 mm lens, at ISO 100. All that data is in the manual, you can get a PDF of the manual off of the web, then check into this before you ever buy the flash. You need to pay close attention to the units that are specified with the guide number. This flash is called the 580 EX because its highest guide number is 58, but that's referring to 58 meters.
A manual, fortunately, lists guide numbers in both feet and meters and they just stuck the zero on here because 58 EX doesn't sound as cool as the 580 EX, but very often the name of the flash would be an indication of the guide number. Maximum guide number. Now as we've seen flash brightness is a effect by aperture. So shooting with a wider aperture effectively gives you flash greater range. Using the guide number you can determine the effective range of the flash at any given aperture. For example if I'm shooting at F8, I simply divide the appropriate guide number by the aperture to find out the effective range of the flash.
In this case, I find that I have a range of 190, divided by eight, or 23 feet. It's quite a bit less than the full range of the flash. In comparison, this Canon 430 EX which has a guide number of 141 when shooting at 105 millimeters at ISO 100, at F8 this flash will have an effective range of 17 feet. So at F8, the range in these two flashes is only 6 feet. Now that may not seem like much, which may lead you to wonder why you should spend the extra money on the bigger flash.
But as distance increases, the difference in flash range will grow significantly and you can calculate that from the guide numbers. You can also use the guide number to calculate a good aperture for a subject at a given distance. That is, an aperture that will ensure good flash coverage. Now, the only time that you might need to do that is if you were shooting with manual flash. To be honest, I never think about guide numbers while I'm shooting, but that's simply because of the way I shoot. It's also because I shoot digitally, I can try a shot and review the results on my camera. If you are shooting film, these guide number calculations are often the best way to ensure that you are not wasting film.
Still it's good to know what guide numbers are, and what the guides numbers tabled are in your flash manual, and they are definitely handy when flash shopping if you want to get a sense how a flash power corresponds to range. Power is not the only different between one flash and another. There are other features that you'll want to consider. But if you are considering of different power, you might want to do some simple guide number calculations to get a sense of the range differences between differences at different apertures and ISOs.
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