Nodal point — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
The eye is a picture surface too
A retinal sphere behind an off-centre nodal point takes every measurement this collection puts to a lens or a screen, and answers all of them. It is the equal-area fisheye to within 110 micrometres of retina rather than the equidistant one everybody draws it as, and a flat picture at its own correct distance leaves the identical arc on it, to 1.4e-14 degrees.
The hole a rig cannot fill
A two-lens spherical rig covers every direction between its two lenses and still cannot see 0.470 per cent of the sphere directly beneath it, reaching 12.8 degrees from straight down — its own tripod, standing exactly where neither lens can look. No arrangement of lenses removes it, because it is not a gap in coverage; it is the rig occluding itself.
Named alongside it
The objects these essays reach for when they reach for this one.
Entrance pupilViewing distanceAnisotropyArea scaleConformalCoverageEqual-area projectionEquidistant projectionEquisolidFisheyeOcclusionPanorama