Jim Michael |
January 18th, 2006 08:05 PM |
Quote:
Originally Posted by Douglas Villalba
Light drops in intensity in exponential proportions. For example a 100 watts light gives you X/1 lumens at 1 foot distance at 2 feet it gives you X/2 lumens at 3 ft it gives you X/4 at 4 ft is X/8. So a light at 4 ft needs to 8 times stronger than the light at 1 foot to be equal. you would need an 800 watts light to give you the same amount of light of the 100 watts light at 1 foot distance.
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A quick back of the envelope calculation I used to do with flash photography at events was to use "f-stop feet". If you know that your exposure is correct at f-8 and 8 feet then you have half the light at 11 feet and half again at 16 feet, so your flash exposure at 16 feet would be 2 stops open from f-8 or f-4. The inverse square law works for point sources, the falloff isn't the same for broader sources, e.g. with a large diffuser. So, similarly your lumens at 5.6 feet are half that for 4 feet and half again for 8 feet, half again at 11 feet, etc.
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