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Understanding Flash Guide Number
Posted By: dosdan, 11-28-2009, 02:43 PM

In this explanation of interpreting Flash Guide Number (GN) I will use the example of the Sigma EF-530 DG Super flash. This unit has a GN of 53 when used with a 105mm FL lens on a FF camera . This means at ISO 100 & an aperture of f/1 it can correctly illuminate a subject of 25% reflectance at 53m when used with a lens of this FL.

  1. Varying Aperture. Changing to f/2 will drop the max. distance to 26.5m & f/4 will max out at 13.25m. The next full stop down is f/5.6, and it's easy to work out the distance: 53m / 5.6 = 9.4m.
  2. Varying ISO. Changing ISO sensitivity on the same set-up (105mm FL on FF, f/1) will extend the flash reach to:
    ISO 200 is distanceiso100 x 1.4 = 74m.
    ISO 400 is distanceiso100 x 2 = 106m.
    ISO 800 is distanceiso100 x 2 x 1.4 = 148m.
    ISO 1600 is distanceiso100 x 2 x 2 = 212m.
  3. Varying FL. A 70mm lens on an APS-C will have the same FOV as a 105mm on a FF, so the flash with a 70mm lens on an APS-C camera (1.5x crop factor) with an f/1 lens at ISO 100 would reach to 53m. A 200mm lens on the same camera would still max out at 53m because the flash reflector can't focus the beam any tighter.

    The Flash head on this unit "zooms". The flash reflector moves relative to the flash tube, or perhaps the reflector is fixed and the tube actually moves. This means that the GN changes with FL (up to a limit) but the max. flash power level is still the same.

    GNs for this unit at different FF FLs. 100% power level. (APS-C equiv. FL shown in brackets):

    20 @ 17mm (11mm) with pullout wide-angle diffuser panel
    28 @ 24mm (16mm)
    29 @ 28mm (19mm)
    31 @ 35mm (23mm)
    40 @ 50mm (33mm)
    45 @ 70mm (47mm)
    48 @ 85mm (57mm)
    53 @ 105mm (70mm) or greater

    So the GN increases because the flash angle-of-illumination is decreased (the cone of light becomes narrower) at increasing FLs to match the decrease in FOV as lens FL increases. Once you go over 70mm FL on an APS-C camera, there is no further narrowing. This means, for example, if you use a 135mm lens with the flash, some of the flash's light output is wasted because it falls outside (is wider than) the len's FOV.
  4. Combined. The flash used at f/2, ISO 400, with a 70mm lens on an APS-C camera, would max out out at 53m.
Note: the GN of 53 appears to be specified in a light-coloured room situation, where reflected light from the ceiling & walls may boost the target illumination. So the performance outdoors will probably be less than expected. Thanks to Albert (Canada_Rockies) for that info.

Last edited by dosdan; 02-01-2012 at 01:05 PM.
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