Using Depth of Field to Create Flythroughs

Download clip Broussard.wmv before you begin.

Depth of field refers to the distance from camera where objects will be in focus. Even in a still picture the out of focus areas in front and behind the subject convey depth. I have seen some decent focus pullers in my day but nobody can compete with a key-framed focus pull in After Effects.

In the sample clip from the music video I directed for singer, Marc Broussard, you’ll see the camera pull back from Marc through a hole in a brick wall (where we see a frame of a dancing figure) and then return through a second opening to arrive back to a CU of Marc. I have a companion tutorial, “Shooting for 3D Post” which details the shooting and After Effects 3D techniques used to construct this video, but here we will just “focus” on depth of field.

In photography, there are three parameters that will affect depth of field in a scene: Zoom, focal length (focus distance) and aperture. After Effects also adds “blur level” which I always leave at 100% and let the aperture setting control blur amount.

One of the issues with shooting progressive HD cameras is the lack of depth of field. With a smaller aperture opening than a 35mm film camera, more of the world is in focus even with the aperture wide open. You can throw the background out of focus by zooming in a bit but it’s still not as good as a film camera. If you shoot your elements separately as I did in this video, After Effects allows you to create plenty of depth of field using their 3D camera.



The environment in this video was populated with lots of trees, branches and organic props I captured with my still camera. Camera motion is always enhanced by the amount of stuff it passes through on different planes.

There isn’t much documentation in the manual on this but you can actually see the focal plane from any view and precisely position it to the layer you would like in focus. The depth of field must be turned on to view this visual focus plane.

From the top view you can see the camera path used in this scene. There were only three key frames in this animation. From the top and side view, I adjusted the bezier handles until the camera was positioned to fly through the two holes in the brick wall. The POI remained fixed on Marc.



I key framed the focus distance to accommodate the camera position as it moves from Marc to the brick wall and back. From the top view I slid the numbers in the focus distance setting on the timeline until the focal plane landed on the layer I needed in focus.

If you crank up the aperture value, you will see an increase in the amount of blur outside the focal plane. This can also be key-framed if you want to change the amount of blur at different points in the scene. Of course, render times will increase with the depth of field turned on with a high aperture setting so I usually leave it off until I render. This speeds up my layout time.

This video was pretty conservative with the depth of field but it would be fun to try some crazy focus pulls on different layers in Z space. You can view Marc’s entire video “Marc Broussard Video” from my website.

You might also find Bill's Shooting for 3D Post of interest.

Bill O’Neil
www.chicagospots.com



Discuss this technique or others in the After Effects forum at CreativeCOW.net.