Circles in Perspective
Here is some basic but valuable information about circles in perspective.
Ellipses on the top and bottom of an object do not have the same degree. If you're looking downward at an object, they are skinnier at the top of the object, and they appear fuller as you look lower down on the object.
This diagram comes from the Famous Artists Course. The instructors recommend constructing a transparent box around the lamp to help get the ellipses right. The eye level (or horizon) in this diagram is up at the top of the image, where the lines vanish to a dot. The square cross sections can be subdivided to find the center line, center points, and the square-in-perspective on which each ellipse can be fitted.
A circle at the height of the eye level would be seen perfectly edge-on, and would thus be a straight line.
The same principle applies to ellipses above the horizon. When I drew this round tower, I first established the eye level. It's near the bottom of the small arch-top window in the middle of the picture.
At that level the ellipse flattens to a straight line. I drew the other ellipses becoming progressively fuller as we approach the main ellipse of of the tower's roofline. Having those lightly drawn ellipses in my preliminary drawing helped me place the windows and the courses of stonework.
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