You need an object within 50 cm and background should be 150cm behind the object for the best results. I think the results are best when you have only 1 object close (or a cluster of objects close - but nothing fragmented all over the place).
currently using: Samsung Galaxy S5
2013-2014: Samsung Galaxy Ace
2011-2013: LG Optimus 2X
2009-2011: Sony Ericsson Xperia X1
2008-2009: HTC Touch Diamond
2007-2008: Nokia 6500 Slide