The effect you are referring to is called Depth-of-field or
Bokeh. For example when a character/subject is closer to
the camera its in focus and objects further away is out-of-focus
or visa versa….depending on what you want your viewer to focus
on.
The best method to achieve this is by first having the same
footage on two timelines…. one above the other. Then the
first thing you do is add an effect to the bottom footage
to give it an out of focus look (gaussian blur is what most
would use, but it does not really convey true depth of field)
The fx i use for this is the defocus filter from the
https://www.velvetmatter.com/ plugin.
Then on the top image you use your mask tool to cut out the
subject you want to stay in focus…as soon as you do this
you will see exactly what you are looking for and here is where
you begin tweaking your mask with just the right amount of
feathering…tweak the focus and out of focus areas….
keyframing the mask if there is movement…etc,etc,and in no
time you will get the hang of things.
This might seems a bit tedious, but this is a shot not easy
to imitate in post, because its an in-camera effect.
Steve Rhoden
Creative Director
TNX EFFECTS STUDIOS.
sample bits at
http://www.youtube.com/hentys