The proof a-b: viewing distance and a receding colonnade
Why a row of columns merges to the eye unless the picture is seen through a small aperture
The folio's proof, labeled a-b, holds that a diminished object must be viewed from the same distance, height, and directness as the viewpoint from which it was constructed, or the science will fail. Where a wall will be seen by many different people, Leonardo advises taking the viewpoint at least ten times the size of the object. A row of columns illustrates the point: seen from the center at a, successive columns first touch and then block one another so that the intervals between them are wholly lost, an effect a good perspective reproduces but which reads gracefully only when the work is viewed through a small aperture with the eye at its center.
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The proof a-b: match the eye to the viewpoint
Line a b is offered as the proof. The diminished object must be viewed from the same distance, height, and directness from which the viewpoint was taken, otherwise the science will not have its proper effect.
The ten-times rule for walls seen by many
When the wall will be viewed by a variety of persons, requiring various viewpoints that would make the work discordant and faulty, one should instead stand at a distance of at least ten times the size of the object.
Columns merging in a receding colonnade
Looking down a line of many columns, after only a few intervals the columns touch and then block one another, so the last appears just beyond the next to last and the intervals are wholly lost. A good perspective reproduces this same effect.
The eye at the center of the colonnade, point a
In the lower figure the eye is placed in the middle, at point a, close to the columns. From there the spaces between the columns diminish to left and right until the columns touch and obscure one another.