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Question on the inverse square law
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<blockquote data-quote="Revet" data-source="post: 249163" data-attributes="member: 17612"><p>I was playing around with my flash, testing the inverse square law and I got results different from the theory.</p><p></p><p>equipment used: D3100 (D7100 is on my wish list!!!), SB700 Speedlight, </p><p></p><p>settings used: manual setting, Raw, 1/200 shutter, f/8, Iso 400, manual flash setting. The auto ISO was turned off so the camera couldn't try adjusting ISO.</p><p></p><p>1st shot taken at 8 feet, flash power 1/16</p><p>2nd shot taken at 16 feet, flash power 1/16</p><p>3rd shot taken at 16 feet, flash power 1/8 (one stop increase)</p><p>4th shot taken at 16 feet, flash power 1/4 (two stop increase)</p><p></p><p>According to the inverse square law, when I move from 8 feet to 16 feet, I have lost two stops of flash power so the 4th shot should be close to the first. What I am finding is that the 3rd shot (one stop increase to compensate for 2x distance) is much closer to the first shot than the 4th shot (two stop increase in flash power). The 4th shot is quite a bit brighter in both the histogram and visually than the first.</p><p></p><p>As an added note, the ceiling is 8 feet (white) but I was using direct flash without any diffusers or filters. Also, I did zoom in at the 16 foot range to match the frame of the 8 foot shot for better comparison.</p><p></p><p>Any ideas as to why I am not proving the Inverse Square Law in this example??</p></blockquote><p></p>
[QUOTE="Revet, post: 249163, member: 17612"] I was playing around with my flash, testing the inverse square law and I got results different from the theory. equipment used: D3100 (D7100 is on my wish list!!!), SB700 Speedlight, settings used: manual setting, Raw, 1/200 shutter, f/8, Iso 400, manual flash setting. The auto ISO was turned off so the camera couldn't try adjusting ISO. 1st shot taken at 8 feet, flash power 1/16 2nd shot taken at 16 feet, flash power 1/16 3rd shot taken at 16 feet, flash power 1/8 (one stop increase) 4th shot taken at 16 feet, flash power 1/4 (two stop increase) According to the inverse square law, when I move from 8 feet to 16 feet, I have lost two stops of flash power so the 4th shot should be close to the first. What I am finding is that the 3rd shot (one stop increase to compensate for 2x distance) is much closer to the first shot than the 4th shot (two stop increase in flash power). The 4th shot is quite a bit brighter in both the histogram and visually than the first. As an added note, the ceiling is 8 feet (white) but I was using direct flash without any diffusers or filters. Also, I did zoom in at the 16 foot range to match the frame of the 8 foot shot for better comparison. Any ideas as to why I am not proving the Inverse Square Law in this example?? [/QUOTE]
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