As we saw in Aperture settings for flash, the simplest method of determining the correct aperture for proper exposure when using manual flash is to use the exposure calculator that is supplied with most electronic flash units. It is generally found on the top or back of the flash unit as an integral part of the unit. To use it, you must set it for the speed (ISO number) of the film you are using or that you have set for your digital camera's sensitivity. There is usually an arrow, pointer or mark that must be lined up with the ISO number. Then, you read the correct ƒstop opposite the flashtosubject distance on the dial or index.
Remember that flashtosubject distance (not cameratosubject distance) is the determinant, so if your flash is not mounted on your camera (see Offcamera flash) or if you are using bounce flash (thereby increasing flashtosubject distance), you will need to use the actual distance the light travels to reach the subject.
GUIDE NUMBERS
You can also use guide numbers for determining the required aperture for correct manual flash exposure, something you may wish to do when you find your images are consistently over or underexposed when you have been using the flash's exposure calculator, or when your flash does not have an exposure calculator.
A guide number (GN) is an indication of a flash unit's power (how much light it gives off), and is used in a simple mathematical formula to calculate aperture. The formula is GN/distance = ƒ/stop. It is simply the product of a distance and an ƒstop that gives correct exposure with a certain film speed and flash unit. (Guide numbers may be quoted in meters or feet, according to which system is used for the measurement of distance.) Sound complicated? It isn't.
Divide the flashtosubject distance into the guide number. That's it.
The result is the correct aperture for proper exposure. For instance, working in feet, if the guide number for your ISO setting (or film speed) and your flash unit is 56 and the flashtosubject distance is ten feet, the correct aperture to use is ƒ/5.6. (56 divided by 10 = 5.6). If your flashtosubject distance changes to twenty feet, the correct aperture is ƒ/2.8. (56 divided by 20 = 2.8).
WHERE DO I FIND THE GUIDE NUMBER FOR MY FLASH?
Guide numbers are provided by flash manufacturers, and can be found in the manual supplied with the flash unit. The flash may be assigned a certain guide number for every ISO setting or film speed, but sometimes only one recommended guide number is provided for one speed (usually either ISO 25 or ISO 100). For different speeds, the guide numbers have to be adjusted appropriately (See "If the ISO setting or film speed is doubled...." below). Guide numbers may be quoted in feet or meters, according to which system is used for the distance measurements.
In feet, a guide number of 64 offers the following alternatives (to the nearest ƒ/number):
Distance  Aperture 2 ft  ƒ/32 3 ft  ƒ/22 4 ft  ƒ/16 6 ft  ƒ/11 8 ft  ƒ/8 12 ft  ƒ/5.6 16 ft  ƒ/4
USE THE GUIDE NUMBER TO DETERMINE FLASH DISTANCE, TOO
Using the formula, you can start with a given lens aperture (when you wish to use a particular depth of field, for instance) and determine the required flash distance by dividing the aperture into the guide number.
IF THE ISO SETTING OR FILM SPEED IS DOUBLED, DOES THE GUIDE NUMBER DOUBLE?
No, doubling the ISO doesn't double the guide number. If you double it (from ISO 100 to ISO 200, for instance), multiply the guide number by 1.4. For four times the ISO speed, multiply by 2. For half the ISO speed, multiply by 0.7. For onequarter the ISO speed, multiply by 0.5, and so on. Other than to change the guide number, the ISO setting or film speed doesn't affect the formula. The formula remains intact and usable as a simple means to obtain the aperture or flash distance required. It basically says, divide the guide number by the distance from the flash to the subject, and the result is the aperture required. Or, divide the guide number by the aperture to obtain the necessary flashtosubject distance.
A GUIDE NUMBER IS JUST A GUIDE
A guide number is just what it says: a guide, not necessarily a completely accurate one, either. Minor variances from flash to flash or lens to lens may require some adjustment to get the best results.
If you have been experiencing exposure dissatisfaction with your flash, you can run an easy check to determine the accuracy of the guide number for your flash, and to come up with a practical guide number for all future use.
To determine your own guide number:  Set up a subject exactly ten feet away from your flash. Measure this distance accurately.
 Set your ISO to 100 or put a roll of ISO 100 slide film in your camera (slide film is best because it has less exposure latitude than negative film and exposure variances are not corrected, as the prints from negative films usually are).
 Make a series of exposures at 1/2stop intervals (ƒ/5.6, 6.8, 8, 9.5, 11, etc.)
 In each different exposure, place a card on which you have written in big, bold characters the ƒstop (aperture setting) that you are using for that exposure.
 Be sure to give the flash unit plenty of time to fully recycle between each exposure  at least a minute.
 If using film, have the roll of film processed normally. It's not necessary to have the slides mounted. In fact, you're better off if you ask to have the film strip remain uncut. If shooting digitally, download your images to a computer for viewing.
 Pick the frame with the best exposure.
 Multiply the ƒstop shown on the card in that picture by 10 (the flashtosubject distance you used for the test).
 As an example, if your best exposure is the frame that was shot at an aperture of ƒ/6.8 (a halfstop between ƒ/5.6 and ƒ/8), then your guide number is arrived at by multiplying 6.8 X 10 = 68.
 Now you have an accurate guide number, 68, for your particular flash when using a sensitivity setting of ISO 100 or ISO 100 film. Use it in future, and all of your flash pictures should be properly exposed. (Just don't forget to allow for the filter factor when you use a filter.)
