fisheyes

Fisheyes are ultra-wide-angle lenses or projection systems that produce images with a hemispherical, highly distorted perspective. They are designed to capture a very wide field of view, typically around 180 degrees or more, resulting in a pronounced bulging of straight lines toward the edges. The distinctive look makes distant objects appear compressed and the scene dramatic, especially at the periphery.

Optical projection and image mapping are core aspects of fisheye lenses. Different fisheye designs use different mappings to convert scene angles into image radii. Common projections include equidistant (radius proportional to angle), equisolid angle (radius proportional to the sine of half the angle), and orthographic (radius proportional to the sine of the angle). The choice of projection affects distortion, brightness falloff, and how straight lines behave away from the center.

There are two main forms of fisheye images. Circular fisheye lenses project a circular image within the frame, leaving the corners unused on a rectangular sensor. Full-frame or diagonal fisheyes cover the entire frame, producing a square or rectangular frame with significant distortion. Most fisheye lenses are fixed focal length, though some ultra-wide zooms exist.

Common specifications place circular fisheyes in the 8–12 mm range on full-frame cameras, while diagonal or full-frame fisheyes often fall around 8–15 mm. They are popular in photography for artistic effect, architectural shots, interiors, landscapes, and in virtual reality or panoramic imaging where extreme wide-angle capture and straightforward stitching are advantageous.

In use, straight lines are not preserved in the distorted areas, except those passing through the center. The central area retains the most natural perspective, while the edges reveal the hallmark bulge of the fisheye view.