jQuery ajax contentType vs dataType

$.ajax({

    url: "data.php",

    type: "post",

    data: {"test": "hello"},

    dataType: "json",

    contentType : 'application/json; charset=utf-8',

    success: function(data) {

        console.log(data);

    },

    error: function(XMLHttpRequest, textStatus, errorThrown) {

    }

});