Visual Plane
every picture states the point it is correct from
Concept

Dome port — where it appears

A spherical window for an underwater housing, designed so that every ray meets the glass along its normal and Snell's law bends nothing. Centred on the entrance pupil it restores the pinhole exactly; off centre it bends rays by an amount that depends on the decentring over the radius and on nothing else.

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

00.50011.50205101520the dome's centre, off the entrance pupil (mm)worst departure from the pinhole it would be in air (degrees)centred: exactly zero6 mm → 0.635°a 100 mm dome in acrylic, n = 1.4910.106° per mm of centring error

The port that is not there

A flat window into water costs a lens a third of its field. A sphere centred on the entrance pupil costs nothing at all — not nearly nothing, exactly nothing — and six millimetres off centre costs 0.635°.

refraction · Port 00.2000.4000.6000.800204060how far off the axis the ray leaves the pinhole (°)how far the dome bends it (°)10 cm dome20 cm domeboth at offset/radius = 0.060identical to 1e-16°

The dome knows its offset in units of itself

A dome port centred on the entrance pupil bends nothing at all, exactly. One that is not bends rays by an amount that depends on the decentring over the radius and on nothing else, so a ten-centimetre dome six millimetres off centre and a twenty-centimetre dome twelve millimetres off centre are the same instrument, bit for bit. The picture carries the ratio, which means it never carries the radius.

refraction · Port

Named alongside it

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

Entrance pupilcentre of projectionConditioningCritical anglefield of viewFlat portinstrument limitNormal incidenceNot a projectionParallaxPinholeReconstruction

All concepts