Each power level designates 1 stop or 1/2 the output of the level just above it. So 9 is 1/1, 8 is 1/2, 7 is 1/4 and so forth down to level 1 which is 1/256 of the full output. Moving from 9 to 8 then would be equivalent to adjusting your aperture from say f5.6 at level 9 to f4 at level 8 to achieve the same exposure keeping all other settings, modifications and distances the same.
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