How does zoom on flashes work?

I'm planning on buying the Nikon SB-700 and I would mostly be using it with my Sigma 70-200 f2.8. And it says that the SB-700 has zoom from 40mm-135, would that affect my photos a lot for the extra 65mm, if I would be fully zoomed it on my lens?

Inside the flash, the reflector/flash tube moves in and out to change the pattern of the flash. If you have a mag light, its similar to rotating the flash light head between spot and flood.

The fact that the flash ends at 65mm means the light will cover more than your lens will at 200mm, which is OK.

But, flash units have a limited distance they can work. Light falls off according to the inverse square law, which states that for a 2x distance, there's a 4x loss of light. So light diminishes at an exponential rate.

If you compared the intensity of the light at 10ft and 20ft, the light at 20ft would only be 1/4th as bright. And at 40ft, only 1/8th as bright, and so on. This is called 'light falloff" and rapidly diminishes the distance at which the flash can work.

Having the ability to shape the light by focusing it into a narrow beam will help to extend that distance, but since it stops at 135mm, you can help it out by using a snoot (or make one with a piece of cardboard).

But the flash will only work for about 100ft give or take, under the best conditions, so don't expect to illuminate distant objects with it.

http://www.althephoto.com