1. A three dimensional display system comprising: a display ( 207 ) for presenting a plurality of views of a scene, each view corresponding to a viewing direction for the scene; a rendering unit ( 203 ) for generating an image for each of the plurality of views, the rendering unit being capable of generating differentiated images for neighbouring views containing differentiated image data providing a three dimensional depth effect when neighbouring views are viewed by different eyes of a viewer; means ( 209 , 211 ) for generating a user presence estimate in response to an attempted detection of a user in a viewing area for the display system; and modifying means ( 213 ) for modifying the differentiated images for the neighbouring views to adapt the three dimensional effect in response to the user presence estimate.