There have been some attempts to say that manipulation in nature photography is unacceptable, and to define what "manipulation" is:
http://www.rps.org/news/2014/may/nat...inition-agreed
And in photojournalism, the furore in the World Press Awards last year included intense discussion about how much manipulation is acceptable:
http://www.bjp-online.com/2013/05/wo...rch-for-truth/
I broadly agree with the first attempt at definition, while remaining uncomfortable about how much manipulation to enhance the drama in a scene seems to remain acceptable in photojournalism. IMHO both issues share a common basis in the need to present some form of "truth", but I think the outcome in the latter (and the link I give above is not a final summary of the outcome) gives more scope for manipulation of the emotions of the viewer than is acceptable to me.