Is there any harm in buying a knockoff speedlight? I don't have the $100+ for a genuine Nikon speedlight and I am noticing some cheap ($30) speedlight knock-offs for sale. What would I be loosing versus buying a genuine speedlight? 1. Would it not be as controllable (it is TTL)? 2. Would the light be off white (it lists it as 5600K, but I doubt that )? 3. Would it eat lots of battery life? 4. Would it not be as bright? Also, I recently got an extension tube. When I use it with my 70-210, I have to zoom to focus. What is my effective focal length going to be like? The centre of the range? The widest point? How many mm in extension tube would I need to get 1:1?