Flash and settings question

[Kinz]

I've got a Nikon D300, a Nikon SB600 flash, and a Sigma 18-125 lens. I need to take photos inside a small auditorium of people on stage at the end of a dancing thing. No photos while dancing, but afterwards, take pictures of people accepting awards. They will be on stage, I will down below where the seating is. I'll need to zoom many times maybe 70 to 100mm. I want to make sure people are in focus and that the light looks correct. My question is that I always thought that your shutter speed should double the lens length. So if I'm shooting at 100mm, my shutter speed should be 200. But if I'm using a flash, it's automatically at a 60th. Won't I have blurred shots. What can I do, or what am I missing in my thought process? Also, how can I make sure the color balance is good? Thanks for any help.

 

[tirediron]

What your missing is the flash duration, somewhere in the 1/5000 - 1/10000 second which is what your exposure will be based on, and that will freeze all motion. Alternatively, you could use your flash in manual, shoot at 1/250 and adust your aperture to suit.

 

[KmH]

But if I'm using a flash, it's automatically at a 60th. .

That's just a default setting for P and A mode use, and you can change it.

See pages 288 - 299 of the D300 user's manual (Custom Setttings - menus e1 - e7)

The approximate flash duration of a SB-600 set to full power is 1/900 of a second - see page 88 of the SB-600 user's manual.

As tirediron mentions, when using flash shutter speed only controls the motion stopping of ambient light.