underwater Photo Course :: (2) Equipment Guide :: Underwater Camera Housings :: Underwater Strobes :: underwater Strobe Power Rating

This number corresponds to the F-stopStandard unit of exposure. A means of expressing the illuminating power of a lens regardless of its focal length. (apertureThe variable diameter hole used to control the amount of light passing through a lens. setting) required to get the correct exposure at a particular distance, and with a particular film speed. The standard adopted by most manufacturers' brochures is the land guide number (which cannot be used for U/W calculations) and is in metres with 100 ISOInternational Standards Organization (ISO). The method of film speed (sensitivity) classification developed for the American Standards Association (ASA) speed.

To determine the accuracy of the manufacturers stated guide numbers we measured the output of all the strobes tested on a Minolta Flash meter IV which has a known accuracy of one tenth of a stop (far more accurate than the shutterThe shutter is an integral part of all cameras. It opens to allow light through. The greater the range of shutter speeds a camera has the better. of your camera!). You may not be surprised to see discrepancies between claimed, and actual, guide numbers that we discovered on our test!