Follow focus inaccuracies

[Kylekc239]

So I got a follow focus with hard stop markers for my Canon 70D using the kit 18-135 lens.   I noticed that when I set the hard stops at the place they need to be for my two focus points to be in focus, the focus has changed and the hard stops are not in focus anymore. I've read this has something to do with DSLR's and the autofocus feature of the lens's. Even in manual focus mode this happens. So now the question is, is there a lens that I can use on my 70d that is a full manual focus lens? Has anyone else noticed this when using a follow focus system? I can use my monitors focus peaking to judge by eye when to stop the pull but I'd much rather use those hard stops on the follow focus.

[Waddizzle]

Your lens is a varifocal lens, not a parfocal lens.  You've discovered why some people use a follow focus on the focus ring, too.

https://en.m.wikipedia.org/wiki/Parfocal_lens

What does that mean, and what is the difference?  It's explained fairly well at the above link.  Basically, a parfocal zoom lens will maintain its' focus as you vary the focal length.  A varifocal zoom lens does not maintain focus as you vary the focal length.

A parfocal zoom is more complex to build, which makes them far more costly a varifocal lens.