Monochromatic Abberebation

Monochromatic aberration
1) Spherical aberration
The focal length of a lense for marginal and para-axial rays varies because of the thickness of lense. It os maximus for paraxial and minimum for marginal rays. This inability of lense to focus all rays toa single point is called spherical aberration fp-fm is measure of spherical aberration
Removal
a) If two lenses are kept at a distance equal to difference in their focal length spherical aberration will be minimized
b)If two(convex and concave) lense are kept in a suitable combination spherical aberration can be removed
c)By using Stops for marginal or for para-axial.

2)Comatic aberration
Spreading of image in the direction perpendicular to principal axis is called coma. The inability of a lense to focus all the incident light on a vertical plane from the object placed outside the axis is called comatic aberration
Removal
a) By using stop chromatic aberration can be minimized
b) By using lense which obeys Abbe's sine law

3)Astigmatism
The inability of a lense to focus all the incident rays in vertical and horizontal plane results spreading of image parallel to principal axis is called astigmatism
Removal
1) By using stops

4) Distortion
If a rectangular object be place infront of a lens, image will be formed either in barrel shaped or pin-cushioned shape. This defect is called distortion.
Removal
1) using stops
2) using suitable combination of two conver lense

Curvature of field
A lens has variable focal length for marginal and paraxial rays. Due to this image will be formed in a curved shape and called as curvature of field