Equal figures on a semicircle; the twenty-times rule
Figures equidistant from the viewpoint f, and shaping the perspective screen like the wall
If viewers cannot be held to a single viewpoint, Leonardo advises drawing oneself backward while laying out the work until the eye is at least twenty times the greatest height or width away, so that changes in the observer's position are barely detectable. A semicircle with center f shows that figures placed on the arc d-b-e are all equally distant from f and should therefore be of the same size, even though the perspective construction makes the one at b appear smaller than those at d or e. He adds that foreground objects should be rendered in their proper size and that the perspective screen should always be made in the same shape as the wall to be painted.
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The twenty-times rule of retreat
If those who look at the work cannot be made to stand at a single viewpoint, one should draw oneself backward while figuring the work until the eye is at least twenty times the greatest height or width away. The variation as the observer's eye changes will then scarcely be detected.
Equal figures on the semicircle d b e
With the viewpoint at f, all the figures placed on circle d b e should be made the same size, because each is equally distant from f. Yet the construction below shows this is not so, and why the figure is made smaller at b than at d or e.
Semicircle inscribed within a trapezoid
The same half-circle is set inside a trapezoid divided by two horizontal parallels and three lines converging toward the top, giving a perspective effect. The points d, b, and e on the arc are again marked to test their apparent sizes.
Match the perspective screen to the wall
Two equal things placed at successive braccia away will appear unequal, though this is more disputative than practical when the viewer is nearby. Foreground objects should be rendered in their proper form, and the perspective screen should always take the same shape as the wall to be painted.