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Flash units often have a range of settings that controls the amount of flash emitted. With portable on-camera flash this is normally controlled automatically by either the camera or a sensor on the flashgun. Studio flash are often less sophisticated and have a slider or switch offering half or quarter power. Some of the more expensive models can be adjusted more accurately though a greater range of stops.
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A lighting control that lets you fine tune the output of flash. This comes in useful when space is limited and you can't move lights back and forth or you want to adjust the exposure of one units when used in a set to balance the exposure.
Indicates the energy content of the storage capacitors in an electronic flash. One Joule is the light output given by one watt burning for a second. Therefore a flash unit with a storage capacity of 300 Joules can supply the power to light a 300W lamp for one second. It doesn't take into account any electrical loss between the flash capacitor or flash tube, or the efficiency of tube and reflector.
A flash mode that's used to reduce the power of the output to quarter or half power. Some of the more advanced flashguns can reduce the output to 1/64 power which is useful for macro photography.
Indicates the output, or power, of an electronic flash. You can use the guide number to work out the aperture or flash to subject distance by dividing either into the guide number.