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How far away can the player be with you still getting good light on them
Once it gets dark I usually put away the 600mm and switch to my 200/1.8L+1.4xTC for 280mm (f/2.5 available, but I usually click it up to f/2.8). If they're close enough to sufficiently fill the frame, then they're usually within flash distance. They need to be closer if you're shooting at xsync, trying to overpower ambient. Hypersync with multiple flashes at high ISO is more flexible and more recoverable in PP if necessary. But as a rule of thumb, I don't try to reach past midfield. I move to the endzone when they're in the red zone (20 yards), so that's usually with range.
have you ever been told the flash hindered the player, or better yet have you been told it was of no consequence?
There are hundreds of threads in the past 10 years on this, so I don't want to revisit this tangent in depth here. I've been shooting this team with flash for several years. And I've shot hundreds of games at 77 different schools in several different states.
I've checked with officials, ADs, coaches, and players. Not a single concern or comment (other than "great shots last week" or the players saying "we're going to run this play your way, so get ready").
YMMV. If the guidelines, officials, ADs, or coaches in your area prohibit or discourage the use of flash, or if you have concerns or preferences against the use of flash, then don't use it.