Navigate camera with MMB - center?

I don’t know what to look for in the manual. Perhaps someone can give me a hint into the right direction:

In 3D View you can control the user camera with MMB. Around which point it turns? And how can I change it?

Sometimes I got the situation that controlling the camera this way turns around a point very outside what I am seeing so it gets difficult the position my view the view I want it.

For looking through the camera, in the properties panel try enabling ‘Lock Camera to View’

For navigating in the 3d viewport try some of the View Manipulation options in the User Preferences