In the render window the changing depth of field works when the camera is animated but it does not render that way. We are stumped.
Basically here is what is going on. I setup an animation with some particles that have an emission property, sparks. I've got a camera with with an fstop set to 0.5 and a 35mm lens. I set the camera to focus on an empty object. Then I animated the empty object moving toward and away from the camera.
Now everything works perfect so far. I can animate a single frame as an image from any point in the animation and everything looks great. I get just the right amount of depth of field and distance camera blur, according to the distance of the empty object from the camera and the frame number I am on. It also pre-renders in the preview window just as expected. Perfect.
However when I go to render the animation the focal distance should follow the empty object, but it remains constant. I have set many key frames for the empty object, moving far and close to the camera. All the frames render out as if the empty object never moves from the first key frame.
I've been doing this in 2.78a, 2.78b & 2.78c. Windows 7 & Windows 10.
As I said, I am doing another test render now to see what the outcome will be. I changed the camera object to display the depth of field in the preview window, and I changed the camera object do use the fstop in the preview mode.
I did recreate this whole scene without the particles using monkey heads and following another different youtube tutorial on FStop & depth of field and created a test render and the animation came out just fine. The focal distance followed the empty object without issue. So now that I know it works, it's just a matter of making it work in the main scene that I'm trying to render.
More updates forth-coming. If it's still an issue I will post the blend file, and maybe we can get to the bottom of this.