The most pwerful flash I'm aware of is the Metz Mecablitz 70MZ-5. At that focal length it has a GN of 70 at 100 ISO. If you cranked the ISO up to 1600 then you could potentially get a range of up to 100m with a 300mm f/2.8 lens.
I've never heard of anyone using a flash like this for footy though. Can't you just use the ambient light or is your lens too slow?
Christian Wright; Dip Phot
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