In order to understand a perspective drawing, a picture plane must be imagined between the observer and the subject. This plane has a constant right –angle relationship with the central visual ray. Thus, so when we draw an object which is above, below or straight ahead, we imagine viewing it through a picture plane which is perpendicular to our central visual ray in the cone of vision.

Why appearances differ from reality:

By applying the notions of lines of sight through a picture plane to simple views of pencils of equal length we can more precisely define the visual basis of Diminution, Convergence, Forshortening and Overlapping, and explain diagrammatically why appearances of objects appear different from reality than in drawing.

Lines of sight applied through picture for Diminution

Diminution:

Fig.94: looking at pencils kept in parallel positions which are vertical to each other.

Objects of equal size appear to reduce in size as their distance from the observer increases. The pencils here are not exactly in line but are kept vertically. Pencil 2 appears smaller than pencil 1; this is because of the way the lines of sight leading from eye to object intersect at the picture plane.

Fig.95: looking at pencils kept in line with each other.