Guide number - useable light

Mike Gallagher · 09-01-2014, 08:14 PM

A flash's guide number is given as 35 - 35 metres with ISO 100, 35mm. (I'm assuming that's a 35mm focal length lens.) Doubling the distance gives less than double the light at that distance unless the ISO is doubled too. Is that right?
What happens to the light? Is it always enough for a proper exposure at those calculated settings and distances? At what point does the light diminish to below a useable illumination? Because of the scale of diminishing returns it must happen rapidly when it happens.

clicksworth · 09-01-2014, 08:34 PM

The Inverse Square law applies, with only one quarter the light at twice the distance. This is why it fits so nicely into the F number calculation. The light becomes unusable when you can't open the lens aperture any wider.

Ole · 09-02-2014, 04:52 AM

To add to the above response, the guide number changes with the focal length of the lens in use with such flash units that have a zoom head (provided that the flash actually zooms (or is manually zoomed) to match the focal length of the lens).

A flash will not zoom automatically with K, M and A series lenses.

Mike Gallagher · 09-02-2014, 06:18 AM

Thanks for that. I think I got it.

09-01-2014, 08:14 PM	#1
Mike Gallagher Junior Member Join Date: Mar 2014 Posts: 29	Guide number - useable light A flash's guide number is given as 35 - 35 metres with ISO 100, 35mm. (I'm assuming that's a 35mm focal length lens.) Doubling the distance gives less than double the light at that distance unless the ISO is doubled too. Is that right? What happens to the light? Is it always enough for a proper exposure at those calculated settings and distances? At what point does the light diminish to below a useable illumination? Because of the scale of diminishing returns it must happen rapidly when it happens.

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09-01-2014, 08:34 PM	#2
clicksworth Senior Member Join Date: Nov 2013 Photos: Albums Posts: 150	The Inverse Square law applies, with only one quarter the light at twice the distance. This is why it fits so nicely into the F number calculation. The light becomes unusable when you can't open the lens aperture any wider.

09-02-2014, 04:52 AM	#3
Ole Administrator Join Date: Sep 2006 Location: Copenhagen, Denmark Photos: Gallery \| Albums Posts: 4,408	To add to the above response, the guide number changes with the focal length of the lens in use with such flash units that have a zoom head (provided that the flash actually zooms (or is manually zoomed) to match the focal length of the lens). A flash will not zoom automatically with K, M and A series lenses.

09-02-2014, 06:18 AM	#4
Mike Gallagher Junior Member Join Date: Mar 2014 Posts: 29 Original Poster	Thanks for that. I think I got it.