Need some help with my Mac hard drive formatting...

So I have a mid 2011 Macbook Air, and have been trying to get my hard drive partitioned for boot camp (which I used to have running, but got rid of due to space constraints)

Anyway I cleared 50GB of space, (according to the 'get info' section in Finder) however disk utility (and boot camp assistant) is reading it as having practically no free space, thus wont let me partition the hard drive and coming up with the error "you need over 20GB of free space to continue"

Exbuxij_medium

Note: the same error was occurring when I had over 50GB of free space,

Is there someway to clear the empty space somehow? I've tried 'repair' function in disk utility to no avail, but I've tried everything I can think of, so any help would be much appreciated