Image Planes and Rack Focus

Image Planes and Rack Focus

Any time you can divide a shot into distinct planes of depth
with each plane as its own layer, you can rack focus. All you
need is a focal point to animate and a depth of fi eld narrow
enough to create blur everywhere but the immediate
plane of focus.
Narrow depth of fi eld is created on a real camera by lowering
the f-stop value, which lowers exposure as well. Not so
with the After Effects 3D camera. Its Aperture and F-Stop
settings  affect only focal depth, not exposure
or motion blur. The two settings have an inverse relationship.
F-Stop is the setting more commonly referenced
by camera operators, and yet only Aperture appears as a
property in the Timeline.

After Effects depth of fi eld settings can be matched to a
camera report, provided that it includes the f-stop setting
used when the footage was shot. If so, open up the Camera
Settings dialog (Ctrl+Shift+Y/Cmd+Shift+Y, or doubleclick
on the Camera in the Timeline panel), check the box
labeled Enable Depth of Field, and enter the value.

The key here is to offset at least one layer in Z space so
that it falls out of focal range. Now, in the Top view, set the
Focus Distance (under Options) to match the layer that
will be in focus at the beginning of the shot, add a keyframe,
then change the Focus Distance at another frame to
match a second layer later in the shot .
A static focus pull doesn’t look quite right; changing
focus on a real camera will change the framing of the shot
slightly. To sell the example shot, which starts on a view of
the city and racks focus to reveal a sign in the foreground,
I animate the camera pulling back slightly, augmented by
a nice shift that then occurs in the offset planes of focus.