LESS power......not more.

Better yet, put an ND gel on the light source. They are available in many densities and all you need to do is tent it over the flash tube, a couple of small clamps will fasten it to the speed ring. And as they say "Bob's yer uncle".

BTW, have used this technique this for years and it works just fine. Make sure the gel is not touching the flash tube and you'll be good.
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35 to 4 x 5 - NPS Member