thks for all the comments.
was think of achieving bokeh or blur background.....sorry i'm a newbie....
You might want to describe the situation and effect you're trying to achieve, and may be even post an example.
If you're expecting a 50mm to give you a full body portrait with a totally blurred background, you're gonna be disappointed. But a half-body shot, or certainly headshot, presuming sufficient separation from the background, you should have a very nice bokeh at f/1.8 or even smaller apertures. And for full-body portraits, it's still a fine lens, you just need to be careful on background distance/content.
Agree with the previous poster that you may want to read up on depth of field as well.
If you looking for full-body shots with nice creamy bokeh, then you should be using a longer lens (say, 300mm f/2.8) and make sure you have sufficient separation (distance) between your background and subject. Maybe that's what you're meaning by "effect".
But for headshots, or half-body shots, 50 f/1.8 will do it.
This image was at 52mm (on a 24-70) and f/2.8. The background is a curtain, and I think it's sufficiently out of focus. Not at great example of "bokeh" per se, but it's an example of the depth of focus one can achieve at that focal length even at f/2.8 (though this is on a full-frame 1DS2).
Eric