How can I define the depth of my world? I created an island with clouds hanging in the sky somewhere behind the island and a camera that approaches the island and moves around it, before moving away from it. When my camera begins approaching the island, the clouds are  not visible. When the camera is right in front of the island, the clouds are all visible. And when the camera moves away from the island, the clouds dissapear again. What is going on there? I think, it has something to do with the depth of my world or the depth of what the camera can see. Thanks for help!
Probably clipping. Both Viewports and Cameras have near and far clip settings. For the Viewports, they can be found in the 3D View Properties Region, 'View' panel. For cameras, they are in the Properties editor, Camera Data tab.