So I have seen many a thing for push pull focus on DSLR video going around at the moment. The commercially available ones can get really pricey, or be pretty competitive like this one from FocusMaker. I have even seen pretty low fi hacks using can openers.
There are many ideas floating around about how these things can work.
Do you use one? What do you use, and how well does it work? Please comment and let me know what you use and how well it works.