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) {
    }
});