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

Sighting point — where it appears

The fixed place an eye must occupy for a drawing instrument to work, such as a veil's peephole or a string frame's hook. An apparatus fixing it mechanically has no error from a wandering eye; one that asks a draughtsman to hold still has one, and it is measurable.

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

correct from 14 cm, at 160 mm wide12×12 cells · worst 1.57 px

Copying square by square

The taught grid workflow sets a pavement's cell corners out exactly and then fills each cell by eye, and the corners are right while the fill is not — 3.30 px on a picture 690 across at eight cells, falling as the square of the cell. On a wall square to the camera the same fill reads 3e-13 px, which is why the method feels reliable.

construction · Gridcopy a plan, looking down: the object on the left, the frame, the hook on the rightthe hookthe frame, edge-on160 cmplan at 0.25 px per mm · a hook 160 cm behind a 56 cm framecorrect from 46 cm at 160 mm wide

The hook is the centre, and the eye is not

Dürer's string frame projects from a ring of iron driven into a wall, so its centre of projection has a position anybody can measure with a tape. A hook 120 centimetres behind a frame 56 centimetres wide makes a drawing correct from 34.3 centimetres shown 160 millimetres across, with its principal point 161 px off the middle of the sheet. The gridded veil projects from a head instead, and 30 millimetres of head moves its marks by 13.27 millimetres.

construction · Instrumentdrawing

Named alongside it

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

Picture planeAffine mapcentre of projectionDemonstrationDrawing systemFocal lengthForeshorteningGround planeHomographyinstrument limitMeasuring pointPavement

All concepts