A number that relates the output of electronic flash when used with a particular speed film. The higher the guide number, the more the light output. Guide numbers, or GN serve as a way to calculate aperture when shooting flash in manual exposure mode. Dividing distance into guide number gives the aperture: For example, a flash with a guide number of 56 (with ISO 100 film) would give a correct exposure at 10 feet with an aperture of f/5.6. With the state of today s automatic exposure flashes, guide numbers today are mostly useful for comparing the relative power of one flash to another.ritzcamera
A number which serves as a guide to proper exposure when using flash. Also known as Flash factor. The number is based on a flash unit s light output and a digital camera s ISO sensitivity setting or a traditional camera s film speed. When the guide number is divided by the flash-to-subject distance, the correct aperture for proper exposure is determined. Guide numbers may be quoted in meters or feet, according to which system is used for the measurement of distance.
Guide Mode provides instant in-camera guidance that shows you step-by-step how to change camera settings to achieve the picture you want. To use you select a Guide Mode setting that matches the scene you are shooting, and let it assist you, so you learn while obtaining the desired results.
The power output rating of a speedlight flash unit.
Often written GN, describes power rating of an electronic flash, with higher number indicating more powerful flash. Expressed, for example, as GN 30 at ISO 100.
The power of a flash in relation to ISO film speed. Guide numbers are quoted in either meters or feet. (To convert from meters to feet, multiply the metric number by 3.3). Guide numbers are used to calculate the f/stop for correct exposure as follows: f/stop=guide number/distance.