Shortcut to lock the screen on a mac

Sometimes i don’t want to put my macbook air asleep because of some job that is running. But i have to leave the room so i need to lock it.
So there is a shortcut for locking the screen:

Shift+Crtl+Eject

This works on the most macs, but the macbook air has no Eject button on the Keyboard. So on systems without an Eject button you have to use:

Shift+Crtl+Power