Is it possible to set the focus for different layers in blender? I’m not very experienced with layers…
Are you referring to a depth of field effect?
“The distance from the camera” is referred to as Z-depth and it is a factor which you can use (perhaps using a “curves” node to map it) for a variety of purposes including blur. But, “depth of field” is a direct implementation of this photographic lens effect.
Well, I know about depth of field, but I just want specific objects in a separate layer to be blurry.
One way is to use Render Layers in the Compositor …
https://docs.blender.org/manual/en/dev/render/post_process/layers.html
This lets you select-out different portions of the scene (identified by the “layer buttons”) for distinct treatment such as blurring.
In that case I wouldn’t bother with depth of field at all. Just make sure that your objects are in different layers, and then add blur nodes to each render layer in the compositor.
You can animate the blur nodes too for focus-pull effects. I used that technique for this video.
Hm, okay. The compositor nodes will automatically remain blurry through the animation if I want them to, right?
Hm. Okay, thanks! This looks really useful! 
Yeah, they’ll do exactly as you wish. As I mentioned you can even animate them.