WhAT IS "RECTILInEAR," WhAT IS A "FIShEYE"?
When a lens projects a three-dimensional scene onto a
two-dimensional piece of film, not all geometric properties
of the original scene can be maintained. This is essentially the
same problem as mapping the shape of the continents of our
three-dimensional globe onto a two-dimensional map. The
choices of lens design, focal length and distance to the subject
determine the character of this mapping, which is commonly
referred to as perspective, one of the cinematographer's most
important tools. For wide angle lenses, the lens designer must
make a choice between a rectilinear or a fisheye lens design,
with different consequences for perspective. The most obvi-
ous differences can be seen by how straight lines and objects
at the edge of the frame appear.
Since the human eye judges distance by the way elements
within a scene diminish in size and the angle at which
lines converge, most lenses are designed to duplicate those
"natural" geometric relationships on film. This is called a
rectilinear perspective, and to achieve it the lens will stretch
the image so that vertical, horizontal and diagonal lines that
we perceive as being straight are reproduced as straight lines
on film.
There is, however, a limit as to how wide a lens with a
rectilinear perspective can be, based on the limited amount
of space available in front of the camera, and on various
optical problems that get increasingly unwieldy as the angle
of view increases. The 114° horizontal angle of view (for the
Super 35 format) of the Ultra Prime 8R is already at the limit,
making it a unique and unusual lens in the cine, video and
still photography fields.
Because it is so difficult to design an extreme wide angle
lens with a rectilinear perspective, many extreme wide angle
lenses are designed as fisheye lenses. A fisheye lens can have
a wider angle of view than a rectilinear lens, but it maps the
scene to film differently than we perceive the world around
us, because the focal length is actually changing within the
image. The farther a straight line is from the center of the
frame, the more it will be rendered as curved, with objects at
the edges of the frame heavily distorted by a fisheye.
A rectilinear wide angle lens on the other hand renders all
straight lines in the subject as straight lines in the image. To
achieve this, though, there is linear stretching applied to the
image that increases as an object gets closer to the frame edge.
This effect tends to exaggerate perspective, i.e. it will make
rooms appear larger than they are, enhancing the illusion
of depth, or making speed appear greater if the camera
moves. However, a circular object, like a ball or a person's
head, located near the edge of the frame will appear to be
somewhat enlarged and will have an oval shape.
Two different perspectives
The most obvious difference between rectilinear
and fisheye lenses can be seen by how straight
lines and objects at the edge of the frame appear.
Neither fisheye nor rectilinear wide angle lenses
represent reality in quite the same way as we see
it. They provide two different ways to manipulate
perspective, to change the illusion of space and
distance.
Left: Ultra Prime 8R (rectilinear)
Right: 8 mm Fisheye lens