I have the 70-200 f/4 non is and I just picked up the f/2.8 non is, I have 7d & 5dmkii, the f/4 is a great lense. Do you really think the f/2.8 is necessary? I haven't used the f/2.8 much but I haven't been wowed. Thinking about returning to B&H and getting the fisheye zoom. Does anyone have an opinion on this?