I have a question about close up filters. The one I have seen the most info about is the Canon 500D. I am interested in using one with my 70-200 2.8, which has the 77mm thread. My question is this: Can someone explain the differences in X factor with the 500D vs a set of filters with +1, +2, and +4. The descriptions on the set, in this case a Hoya, say that they can be stacked, ie. +2 and +4 = +6. Any help would be appreciated.