As far as I know, the guide number is the distance the flash would reach at full power. Unless otherwise stipulated I believe it assumes ISO 100 and f/1. In the case of the built in flash on the D200 that's 12 meters or 39 feet (manual 13 meters and 42 feet).
So if you increase the f-number the distance will shorten. Conversely if you increase the ISO the distance will lengthen.
The guide number isn't the distance, it's like your formula says, GN = f/stop x distance or f/stop = GN/distance. 12 is the GN when using meters as the distance. 39 is the GN when using feet as the distance. The GN would double with doubling the ISO. Does GN really have a unit?
I'm sorry to disagree with some others here, but flash guide numbers do not have units - it is a just a number. This point was drummed into us in my first formal photography class. The guide number uses distance to determine an aperture as a starting point for exposure. Because the guide number uses distance to compute aperture, it is necessary to have a different guide number for a different unit of measurement. Most of the time a flash will list two guide numbers - one for distance measured in meters and one for distance measured in feet. Unfortunately, many manuals/instructions list the numbers something like "12 meters/30 feet" and so it has become a common misunderstanding that the guide number itself is expressed in units.
So... When the manual shows "Guide Number 12/39 flash (ISO 100, m/ft)" they are saying: 1. If you measure the distance in meters the guide number is 12; or 2. If you measure the distance in feet the guide number is 39.
Richard's explanation is what I was getting at. It's just a number or constant used in an equation to determine the f/stop at a particular distance. It varies depending upon the unit of measurement you're using for the distance (that's where the m/ft comes from), but it doesn't have a unit of measurement itself.
>Yeah, I think Rick is correct about that. Did we ever answer >the OP's original question?
Actually yes.. My intitial misunderstanding was the way Nikon expressed the guide number rating of the pop up flash.
When they stated that there was a guide number and then listed feet and meters I was thinking that the numbers they were noting were the feet /meters that the guide number was calculated but somehow they failed to list the guide number. I was not aware that they were indeed listing the guide numbers expressed for both feet and meters. So it was essentially my misunderstanding of Nikons terminology.
However, another question comes up from the posts.
I have read that a guide number of 160 is twice the power of one that is 80. However I do not undertand how this can be true.
At 10 feet: Guide # of 80 = f8 Guide # of 160 - f16 With two stops of difference in the exposure is not a flash with a quide number of 160, 4 times as powerful as a flash with a guide number of 80 ?
>My intitial misunderstanding was the way Nikon expressed the >guide number rating of the pop up flash.
It's not just Nikon - everyone (that I know of) uses the same convention.
>However, another question comes up from the posts... > >With two stops of difference in the exposure is not a flash >with a quide number of 160, 4 times as powerful as a flash >with a guide number of 80 ?
That is correct. What you read (about GN 160 being 2x the power of GN 80) was wrong. Perhaps the confusion arises because doubling the GN doubles the effective distance, but increases the power by 4x. In my view, GN's are a means to an end - they allow you to work out how far away your subject can be for a given aperture and ISO.
>I have read that a guide number of 160 is twice the power of >one that is 80. However I do not undertand how this can be >true. > >At 10 feet: >Guide # of 80 = f8 >Guide # of 160 - f16 >With two stops of difference in the exposure is not a flash >with a quide number of 160, 4 times as powerful as a flash >with a guide number of 80 ?
This is where I don't understand why some stipulate the guide number is unit-less.
You are correct in that the inverse square law applies and if you double the distance you need four times the power.
So a GN of 160 is twice the distance of a GN of 80 which infers that the flash with the 80 GN is 1/4th the power.
That's correct, and you're understanding the concept. Twice the power would be a guide number that's about 1.4x larger (square root of two). In the pre-automatic flash days (including manual electronic flashes and flash bulbs), this was one of the ways we set the aperture for a correct exposure. Usually there would be a dial on the flash that you could set to a certain ISO and it would then in effect do the math for you. It still works with today's flashes as well when you're in manual mode. Even if you're using automated flash, it's a helpful concept to understand.
>I'm sorry to disagree with some others here, but flash guide >numbers do not have units - it is a just a number.
I’ll be the first to admit; I’m not the sharpest tool in the shed and have had no formal training when it comes to photography. Being self-taught for the most part I realize that I may have numerous misconceptions (although I don’t think this is one of them, but I could be wrong!).
So with that said, perhaps I’m misunderstanding this phrase which is an excerpt from this article over at Nikon’s imaging web site.
“Quote” “The flash guide number (GN) is a measure of the distance at which the flash can illuminate a subject.” “Unquote”
I take that to mean what I defined in my first response and that is the GN listed in the specifications is the maximum distance a flash can reach and assumes an f-number of 1 and an ISO of 100 unless otherwise stipulated.
After all, to be a measure of distance would imply that it is associated with a particular metric otherwise it would be a meaningless number. For example if I tell you I have two flashes, one with a guide number of 40 and another with a guide number of 50, which is more powerful? You then might tell me the one with the GN of 50 at which time I tell you “oh yeah I forgot, the one with a GN of 40 is in meters and the one with 50 is in feet” you would then say “oh in that case the one with a GN of 40 is more powerful”.
So in that regard I believe my first response directly answered the OP, albeit perhaps not clear enough. Of course that is unless I am under a misconception in which case I must ask, “what am I missing here?”
The guide number of a flash is a rating of the power output measured in meters or feet. The number is handy when trying to compare different flashes when purchasing of one.
After the purchase, the guide number is not that useful since the camera and flash displays will assist you in getting the correct exposure for your picture without you knowing or caring what the guide number is.
Just for comparison: d200 Guide Number of 12/39 (at ISO 100, m/ft) sb910 Guide number of 34/111.5 (at ISO 100, m/ft) This means the the sb910 flash will shoot about 3x further.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA
>The guide number of a flash is a rating of the power output >measured in meters or feet. The number is handy when trying >to compare different flashes when purchasing of one.
Unfortunately, though, getting the Guide Numbers in the same form can be tricky. Some brands quote GN for the maximum zoom of the flash, or for higher ISO both of which tend to increase the GN and exaggerate the power of the flash.
>After the purchase, the guide number is not that useful since >the camera and flash displays will assist you in getting the >correct exposure for your picture without you knowing or >caring what the guide number is.
>Just for comparison: >d200 Guide Number of 12/39 (at ISO 100, m/ft) >sb910 Guide number of 34/111.5 (at ISO 100, m/ft) >This means the the sb910 flash will shoot about 3x further.
Of course, the D200 has a fixed flash illumination pattern, so the GN is fixed. The SB910 has different illumination patterns, DX or FX mode, and variable zoom settings. That gives a range of Guide Numbers. The SB900 has Guide Numbers of 13 to 58, all at ISO 100, in metres; I suspect the 910 is similar. See what I was saying about having the Guide Number in the same form?