Explanation

(c) The aberration of lens due to which all the rays passing through the lens are not focused at a single point and image of point object formed is blurred is called spherical aberration. It is reduced by a using two plano convex lenses separated by a distance. If the distance between the two plano convex lenses is equal to the difference in their focal lengths and the incident rays fall upon the lens of large focal length, then this combination is almost free from spherical aberration. In this divided at all the surfaces of the two lenses and so, the spherical aberration is almost removed. This method is used in removing spherical aberration in eyepieces.