Constructing circles in perspective

A circle when viewed at an angle does not look circular but elliptical. For example look at the way the red circle gradually changes to a thinner and thinner ellipse as the disc rotates.

When a circle is drawn inside a square, the circle touches the edge of the square at the midpoint of each side. This enables you to create a guide for your perspective circle since you know that the circle must touch the midpoint of a square drawn in perspective.

To find the midpoints of a perspective square, draw in the diagonals to find the centre of the box and then draw a vertical line through this centre point and a line going through the centre to the vanishing point. The ellipse can be drawn in by hand.

Perspective ellipse templates similar to isometric templates can also be bought. These come in a variety of sizes from 5 - 250 mm at various angles. The perspective of each template varies from fat almost circular ellipses to long thin ellipses.

Using ellipse templates is very easy, every ellipse has a minor and major axis marked on it. So once you've drawn a perspective square, draw in the diagonals. These are the major and minor axis. It then a simple case of finding the correct ellipse to use.