Lens Distortion

Lens Distortion

A virtual camera with a wide-angle view (like the one back
 has a dramatically altered 3D perspective,
but it has no lens to distort objects the way a real camera
lens does. A virtual camera can widen the view area in a
completely linear fashion.

A physical lens curves light to project it properly across the
camera backplate with physical width and height, no matter
how small. To show up properly the refl ected imagery
must be perpendicular to the surface of the lens glass, so
a wide-angle view requires not only a short lens length but
also a convex lens in order to gather the full range of view.
At the extremes, this causes easily visible lens distortion;
items in the scene known to contain straight lines don’t
appear straight at all, but bent in a curve. In a
fi sheye lens shot, it’s as if the screen has been infl ated like
a balloon.

As you refi ne your eye, you may notice that many shots that
aren’t as extreme as a fi sheye perspective contain a degree
of lens distortion. Or you might fi nd that motion tracks
match on one side of the frame but slip on the opposite
side, or proportions go out of whack, or things just don’t
quite line up as they should.
There’s no way to introduce lens distortion directly to a
3D camera, but the Optics Compensation effect is
designed to add or remove it directly on the 2D layer or
stack.  shows this effect in action. Increasing the
Field of View makes the affected layer more fi sh-eyed in
appearance; to correct a shot coming in with lens distortion,
check Reverse Lens Distortion and raise the Field of
View (FOV) value.

This process is not exactly scientifi c. You typically eyematch
the setting, because Field of View doesn’t correspond
to measurable data in the camera report, such as the
Lens Angle.
1. Having identifi ed lens distortion, precomp
the background into a new composition that is at least
20% larger than the plate (to allow room at the borders
to distort).

2. Add an adjustment layer above the plate layer, and
apply Optics Compensation to that layer. With a wide
angle lens, check Reverse Lens Distortion and raise
the Field of View (FOV) setting until all straight lines
appear straight.
3. Add a Beam effect to the adjustment layer (below the
Optics Compensation effect, unaffected by it). To get
away from the distracting light saber look, match Inside
Color and Outside Color to some easily visible hue,
then align the Starting Point and Ending Point along
an apparently straight line near the edge of the frame.
Fine-tune the Field of View setting a little more until
the line is plumb .
4. Precompose all of these layers and set this new composition
as a guide layer. If you can see that
the corner pin is now successful.

5. To complete the shot, restore the original fi eld of view,
including distortion. First create a new comp with
the original background plate (no Optics Compensation)
and the precomp with the assembled foreground
elements.

6. Copy Optics Compensation with the settings added in
step 2 and paste it to the foreground element. Toggle
Reverse Lens Distortion off. The Field of View of the
background is restored, but the foreground elements
match.