Back focus is not really relevant if you meaning holding focus while zooming, as with stills lenses they never hold anyway as they're not designed to. You'll always just use a zoom as a variable prime.
Steve

Thanks Steve.....I was thinking along those same lines. But as you know, strange things begin to happen the further away the back element is from the image sensor. But if it was problematic, then I guess they wouldn't be selling these either.

Patrick McLoad