Consider a plane passing through the object point such as the optic axis lies in this plane. This plane is termed as tangential plane or Meridional plane. Another plane, which is normal to this plane and passes object point o and the optical center is known as Sagittal plane. Now consider a pair of rays OA and OB in the tangential plane. These after refraction intersect at T, whereas the rays OC and OD in the sagittal plane intersect S. Thus at T, though the rays in the meridional plane have focused, the rays in the sagittal plane are not focused and one infact has a focal line called the tangential focal line. Similarly at S, the rays in sagittal plane are focused, but the rays in the tangential plane have defocused and one has a sagittal focal line. Neither at T nor at S, the image is sharp, however in between the two points, there is point, where the defocusing is least and this is known as the circle of least confusion.

To understand the effect better, consider the image of a spoked wheel, which is coaxial with the axis of the lens. At T plane, the rays from the rim is focused well, but spokes are defocused, where as in S plane, the spokes are well focused and rim is defocused.