FL-600R with a Better Beamer for birding ?

Thanks for your answer.  Yes, even the smaller BB is too large for the FL-600R but I can fix that easily.  I'm more concerned about power but I guess I found my answer.

Well... I think I finally found the data I was looking for to calculate my answer. See this article:
http://www.scantips.com/lights/flashbasics1c.html
The GN of the FL-600N is 36 at ISO 100 and maximum zoom (48mm).
I want to calculate the effective distance with the Panasonic 100-300mm f/4-5.6. Since I will mostly use it at 300 mm f/5.6. The formula to find the distance is:
Distance = Guide Number / f/stop
36 / 5.6 = 6.43 m.
So, I'm good up to 6 meters but that's at ISO 100. The formula for different ISOs is :
New Guide Number = (Old Guide Number) x square root of (New ISO speed / Old ISO speed)
The factor from ISO 100 to ISO 400 is X2, X2.83 to ISO 800 and X4 to ISO 1600. Let's use a conservative ISO 400.
6.43 X 2 = 12.86 m.
Well, we're now good to 13 meters. The Better Beamer gives us a factor of X2:
12.86 X 2 = 25.72 m.
If these calculations are correct, I'm good up to 26 meters ! Wow !!! I'm looking for distances between 5 and 30 meters, so I'm right almost there. Well, no problem; let's increase the ISO to 800 and I'm good up to 36 meters.

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Jules Gobeil
Nature photographer - Photographe de la nature
www.julesgobeil.com