I hope that someone has the advice that I am looking for here. I recently bought a new 2x tc, and I find that at the extreme focal lengths I have a little trouble getting my focus as sharp as I would like. Since I have to manually focus at these ranges I would like to buy another diopter for my cameras. My question is how to know which one you need. Can my optomitrist help me with this ? Where do I turn ? If I had a shop near that had them all then this would not be an issue. However I have to know if I'm going to order them from Canon. Any help here folks ?