I was happy to find the telephoto lens I've been using on the Sony VX2000 camera (their VCL-HG1758 lens) seems to work fine on the Canon HF-G10. I am not an expert on optics, lenses, etc. Am I losing anything in clarity, color, quality, etc. compared with other lenses designed for the Canon?

I am already somewhat wary of the long lens as well as long zoom in general. With both cameras when you look closely at the footage, it seems there's a softness, a loss in clarity compared with picking up the tripod and walking closer. I usually shoot from the back row of events, and I hate to setup in the middle rows but that's what I have been doing lately. Why is this necessary? Why am I getting blurry footage at the extreme end of the zoom range?