The appeal of frames within frames is partly to
do with composition, but at a deeper level it relates
to perception. Frames like those shown here and
on the next few pages enhance the dimensionality
of a photograph by emphasizing that the viewer
is looking through from one plane to another.
There is a relationship between
the frame of the photograph and an initial step
in which the viewer’s attention is drawn inward
(the corners are particularly important in this).
Thereafter, there is an implied momentum forward
through the frame.
(30, Part 1, "Frames within frames")