You need a lens motor (or a camera that supports lens ramping, like Canon DSLRs with MagicLantern).
If you want to use a lens motor, you will need a keyframeable controller. Since Focus is not linear, simple A-B moves usually won't work with focus (depending on your subject).
I would recommend that you have a look at the eMotimo Spectrum & Lens motor. They have some examples how to use it on their website.
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