If you apply this iExpression to the Focus Distance of a camera, the focus automatically keeps the Point of Interest in focus.

  • Offset

    Animate this property, if you want to temporarily move the focus away from the point of interest. You can link this to a slider and apply a wiggle to the slider, for example, to simulate some "pumping" auto-focus.

Autofocus on POI iExpression

can be applied to: 
1D properties
iExpression Bundle: 
Download at aescripts.com

What are iExpressions?

iExpressions are After Effects expressions with an easy to use interface.