What is 'Rectilinear,' What is a 'Fisheye'?

[Raman K]

What is 'Rectilinear,' What is a 'Fisheye'?

'Rectilinear' and 'Fisheye' are two different optical designs for wide angle lenses, resulting in very different looks. All modern ARRI/Zeiss wide angle lenses, including the Ultra Prime 8R, are of rectilinear design.

 

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 obvious differences can be seen by how straight lines and objects at the edge of the frame appear.

 

8 mm rectilinear lens

8 mm Fisheye lens

 

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.

 

8 mm rectilinear lens

8 mm Fisheye lens

 

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 lens 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. 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.

--
K.Raman .