I'm looking to create a film with my Nikon d3200 - tips? - 1
I like how cameras change focus during filming - how is this achieved? I also like bokeh effect in the background with a focused forefront - how can I achieve this?
What you are describing is called 'selective focus' and needs a narrow depth of field, which is achieved by getting close to the subject and using a large (low f number) aperture.
Old manual focus lenses are best for this, the cheapest lens you can get with a large aperture is a 50mm f1.4, changing focus is achieved by manually moving the focus from one plane to another by rotating the focus ring, called 'focus pulling'.
It helps if you can fasten a lever to this, then you can easily see the beginning and end point for the focus and move the lever from one point to the other at the speed you want.
The cheapest temporary lever I know of is a Trudeau rubber jar opener.