Point spread function — where it appears
Named by 2 essays across one field — 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.
The disc and the streak
A frame integrates over the pupil and over the exposure at once. Hold the point's depth and the patch is exactly the streak of its centres with one disc slid along it, to 1.8 × 10⁻⁵ of its own width. Let it recede over the same exposure and the disc's radius falls by 3.7 along the streak, and the patch departs from any single kernel by 16 pixels.
Named alongside it
The objects these essays reach for when they reach for this one.
ApertureCircle of confusiondepth of fieldEntrance pupilinstrument limitSensorcentre of projectionChief rayConvolutionExit pupilExposureFocal length