Concept

Vignetting — where it appears

The fall in illumination toward the corners of a picture. Four cosines of it are geometry that no lens design removes — the pupil foreshortened, the sensor tilted, and twice the extra distance — and the rest is the barrel clipping the pupil, which improves on stopping down.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

The centre has an area

Every theorem of perspective follows from a projection through a point, and no instrument has one. Give the pupil a radius and each world point images as a disc — whose centre is the pinhole's mark to 5.7 × 10⁻¹⁴ millimetres, at every aperture, every distance and every field angle. The geometry survives exactly; only the sharpness is spent.

sensor · Aperture

The hole a scene actually sees

The stop is not the centre of projection. Model a 50 mm lens with its stop 18 mm behind the glass and the chief rays from every object distance cross the axis at one point 28.1 mm on the other side of the lens — 10.1 mm from the stop and 1.56 times its size — to 3.6 × 10⁻¹⁵ mm. That point is the entrance pupil, and it is where a picture is a projection from.

lens · Pupil

The corner sees an ellipse

A circular pupil viewed from off the axis is foreshortened by the cosine, so the blur patch a corner receives is an ellipse of axis ratio 0.920 at the edge of a full-frame picture with a 50 mm lens — and the light through it falls as the fourth power of the same cosine, 0.480 stops. Both are geometry, both happen to a perfect lens, and no design removes either.

sensor · Aperture

Named alongside it

The objects these essays reach for when they reach for this one.

Aperture Chief ray Entrance pupil instrument limit centre of projection Circle of confusion Exit pupil Focal length Pinhole Sensor Thin lens Anisotropy

All concepts