Flash Guide Numbers on Flash Units
Guide numbers are a way to compare the power of flash units, but not necessarily a true indication today of all its capability. They were used historically to allow exposures to be easily calculated when flash was used, of course today we have so many other options that few now would regularly perhaps use them for this. However understanding the concepts can prove helpful especially in understanding how far your flash will reach.
Guide numbers look something like, 12/39 at ISO 100 and 17/56 at ISO 200, in each case the smaller number relates to metres and the larger one to feet. The example here is for the inbuilt flash in the D300.
In the simplest form based historically, if you divide the distance between the flash and the subject you get the aperture (f) to set the camera at, so in the example above you can see at ISO 200, with a guide number in feet of 56, if we were 7 feet from the person we were taking a photo of we would set the camera at f8 (56/7) or at 14 feet it would be f4.
The distance is between the flash and subject, not camera to subject, and is applicable to straight undiffused flash. If you bounce it off a ceiling you usually lose about 2 stops.
You could also do a similar calculation dividing the aperture into the guide number to know how far the flash will reach, so if you have a guide number of 40 in feet and a are using f4 you can see your flash will reach 10feet, in a straight line without bouncing.
Although historically we used this calculation, its not really valid today for add on flash units as flash units have zooming heads, so more light is channelled into the area of the subject and less wasted over a wider area. Also we widely use diffusers and other attachments, which have the opposite effect. The guide number can however give us an idea of the comparative power, although not its application.
It may be useful in considering if an inbuilt flash will have any impact when using it for balanced fill flash in bright conditions, or the sort of position you will need to have a remote flash when using it in a similar situation.
For the guide number to double, the power of the flash has to be increased 4 times.
You can compute the guide number for larger ISO values by multiplying the guide number by the square root of 2 (1.414) for each doubling in ISO value. Although this may not appear to work exactly with numbers you see published, this comes about through rounding the numbers published and in most cases we donít know what exact numbers were started with. So if we took the guide numbers for a D300/D200/D80 etc inbuilt flash we would get in feet:-
Guide numbers on current and recent Nikon cameras with built in flash and flash units.
First smaller number is for metres/second for feet
Some manuals and details also show an alternative guide number on some cameras where the metre guide number is one greater when the flash is operated in manual mode.
The SB400* flash is not able to be used as a slave flash or with a commander function as a part of the creative lighting system.
The SB600 and SB800 have zoom heads allowing light to be concentrated, so for example the SB800 manual says that using a range of zoom positions and ISO can cover the range of 2ft to 66ft. However we donít know what this is based upon, and suspect that with wider apertures (lower f) and higher ISOís since the SB800 was announced the current range could be far longer.
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