# Help with guide numbers when using a flash manually

Started Apr 14, 2013 | Discussions thread
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 Re: Help with guide numbers when using a flash manually In reply to Vito_F, Apr 18, 2013

Based upon the chart you supply, the manufacturer of the flash unit presumes that you will be shooting at 100 ASA (100 ISO).

“Guide Numbers” are nothing more than a number the manufacturer supplies to make proper calculations when you are trying to figure out which F/stop to use at which distance. They bare no relationship to how far away your subject is standing from your camera. You have to figure out that distance by using this Guide Number that is supplied. To use these Guide Numbers you must first have a formula to plug them into. The basic formula used is:

Guide Number = Subject Distance x F/Number you want to use (GN=Distance x F/number)

The “Distances” the manufacturer has given you in their chart, are their suggested distances that you might want to shoot at. Unfortunately, they have put them in “meters”. So, you must first convert their “meter” distances to “feet”. Do that by multiplying the number they give you by .3048. Once you have converted their “meter” numbers to “feet”, you now have two parts of the formula. The “F/Stop” to use at that distance, is the number you must now figure out.

Let’s take one of these, and see how it works. Let’s use their number of 105 meters. By multiplying 105 meters X .3048, we come up with 32.004 feet. Round it off to “32” for use in the formula. So far, that gives you:

32 (the GN) = distance x F/number

Now, you must figure which multiple of the F/stop you want to use, that will calculate out to the Guide Number of 32, when you multiply it by the number of feet your subject is standing from your camera. So, “what” times “what” will equal 32? The answer is 8 x 4. Now, plug those two numbers into the formula, and you get:

32 (the GN) = 8 (feet) x 4 (the F/stop to use).

So, if your subject is standing 8 feet away, you must use an F/stop of F/4 to properly illuminate the subject.

You must do this, using each “Guide Number” the manufacturer supplied in their chart, in order to figure out which F/stop you can use at what distance, when you use that flash.

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