In the simplest form of asymmetry, the lens surface appears oval, curving more in one direction than in another other. As Figure 4 shows, parallel rays in red entering the lens in the more strongly curved, or principal, plane focus more closely than rays in blue coming through the other plane. Rays in intermediate planes focus at intermediate distances. An astigmatic lens has two principal axes with different focal lengths, so for the lens shown, rays in the vertical plane focus more strongly than those in the horizontal plane. In a lens with astigmatism, light from a point source will produce a blur, rather than a sharp focus.
Field curvature appears because the object and image distances are measured from the lens' center.
All of the points on that sphere will focus on another sphere of radius v , which is also centered on the lens' geometric center. Distortion appears when the magnification of a lens depends on how far the object point is from the optical axis. Barrel distortion occurs when the central parts of an image are magnified more than the outer parts. Most microscope samples aren't curved, however. The effect of field curvature for flat samples makes it impossible to perfectly focus both the center and the edge of the field simultaneously.