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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. The guide number (or "GN") can be used to compute flash power, subject distance, or camera lens aperture for correct exposure. 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). Guide number is frequently abbreviated as "GN".〔("The Guide Number (GN) article at Photozone" )〕 == Value == The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose film or a digital sensor with the specified sensitivity. ::GN = distance × f-number This simple relationship is well-defined because the brightness of a flash falls off with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number. The guide number represents an exposure constant for a flash unit. For example, a guide number of 80 feet at ISO 100 means that a target 20 feet away will be correctly illuminated with an aperture of (80 = 20 × 4) using a sensitivity of ISO 100. For the same guide number and an aperture of , the light source should be 10 feet from the subject (80 = 10 × 8). Guide numbers do not depend on the focal length of the lens: the distance a flash can illuminate does not depend on the angle of view of the lens. However, some flashes have the capability to "zoom with the camera" and concentrate their light into a narrow beam for use with a telephoto lens. Since the light from the flash is more concentrated, this increases the guide number. Manufacturers typically advertise the guide number for their flashes at the narrowest setting. For instance, the Olympus FL-50 has a guide number of 50m at ISO 100 when set to its narrowest setting, but significantly less when illuminating a wider area. Guide numbers can be given in feet or metres, and are usually given for ISO 100 sensitivity. The guide number of 80 feet in the previous example corresponds to a guide number of approximately 24 metres. The calculations remain the same: a target 6 metres (20 feet) away requires an aperture of (24 = 6 × 4). Thus, when comparing flash units, make sure to compare the guide numbers, measured in the same unit, for the same ISO rating and the same focal length "zoom" setting of the flash. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Guide number」の詳細全文を読む スポンサード リンク
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