Due to curvature of a lens marginal rays meet at a point situated at a nearer distance from lens & paraxial rays undergo minimum & meet at a point situated at greater distance for optical center of the lens resulting in a large no. of blurred images. It can never be eliminated but can be minimized
1. Using stops: By using stops either paraxial or marginal rays are cut off, bringing the rest practically to one focus.
However, in this method, intensity of image formed is poor.
2. Using lens of large focal length
3. Using plano-convex lens such that its curved surface faces the incident or emergent light whichever is more parallel.