Why a Round Tire Can Look Oval
A tire can be perfectly circular and still cast an oval-shaped image across your retina—the light-sensitive layer at the back of your eye. The reason is perspective: when you view a circle from the side, its apparent width shrinks while its height stays about the same. Seen nearly edge-on, the circle can look like a narrow ellipse.
Your Brain Reads the View, Not Just the Shape
Vision does not deliver a finished 3D picture. Each eye receives a flat pattern of light, and the brain uses clues such as shading, familiar object shapes, and how surfaces are oriented to infer what is out there. Because tires are usually round, the brain can recognize the oval image as a circular object turned away from you rather than conclude that the tire itself has changed shape.
But the retinal image still changes as your viewpoint moves. As you walk around the tire—or turn it—the projected oval becomes wider or narrower. Your perception combines that shifting image with the assumption that the object remains rigid, so the change can register as a tire that seems to warp even though its shape is constant.
The Difference Between Seeing and Inferring
This is a small example of a basic feature of vision: what you see is not a direct copy of an object's geometry. It is the brain's best interpretation of changing, two-dimensional input. A tire viewed straight on sends a nearly circular outline to the eye; viewed obliquely, it sends an oval one. The object stays round; its image on the retina does not.
