The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film or sensor sensitivity and angle of view. A higher guide number indicates a more powerful flash.
For example, doubling the guide number means the flash can illuminate an object at twice the distance, or for an object at the same distance can be used at one quarter the sensitivity. Doubling the guide number requires a quadrupling of the flash's power, as the area to be illuminated increases as the square of the distance. (cf. inverse-square law)
Read more about Guide Number: Value, Applications
Famous quotes containing the words guide and/or number:
“The President is not only the leader of a party, he is the President of the whole people. He must interpret the conscience of America. He must guide his conduct by the idealism of our people.”
—Herbert Hoover (1874–1964)
“A child’s self-image is more like a scrapbook than a single snapshot. As the child matures, the number and variety of images in that scrapbook may be far more important than any individual picture pasted inside it.”
—Lawrence Kutner (20th century)