Now i have an iphone4,i want to capture a circle,how can i make it cometrue?[/size]

Try this Screen Grab for Mac(http://macscreenshots.com/screengrab_mac.html), it can capture circular region. Firstly, you need to press Command at the same time you should click "Ellipse" on the capture interface. Plus, a screen magnifier can be used for precise snaps. Then you can grab a circular region very accurately.