The GEOMETRY Workflow - 3D Postproduction: Stereoscopic Workflows and Techniques - page 112

exaggerated for clarity
L and/or R takes on
trapezoidal ("keystone") shape
Figure 4.6 Keystoning.
4.6.2.5 Issue: Zoom Mismatch
Zoom mismatch occurs when L and R present slightly different zoom
characteristics in the form of the contents of one image being larger in frame
than the other ( Figure 4.7) . This mismatch may vary over the course of
the zoom shot because of variations in the L and R image paths during
production. In severe forms this mismatch can “pull” your eyes in a way
that becomes uncomfortable over time.
4.6.2.6 Solution: Correct
If you can determine that the issue is not an error in timing you may need
to blow-up or reduce the size of L relative to R or vice versa. Depending
on the amount of the scale you might get additional issues or artifacts from
the magnii ed image in relation to the unmagnii ed image when viewing L
and R images together.