Semolina Semaphore

Hi there, on a Mac running Mavericks (fully up to date) my camera view (using latest SL viewer) is not working properly. When I press 'alt' and click and move my mouse (camera mode) I can only zoom forwards - there is no way at all of zooming backwards. Moving the mouse backwards does not move the camera backwards, it zooms IN !! I've played with the settings and cannot find any way to fix this. The view just zooms forward until i'm miles underneath the ground and I have to reset my view (F9). Please can anyone help with this?