The depth of field will be less the greater the focal length you use. Therefore if you want to put a background out of focus by having a shallower depth of field then use the zoom from further away and it will do this. This is one reason people use macro lenes for portrait work. You should keep in mind that it could get to be a very shallow depth of field on full zoom therefore you may not get the full subject in focus. Notice how davidy's shots all have a very out of focus background. As above your macro focus on your camera will have a range though which you must stick to.

Message edited on: 05/09/2005 17:14

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html