Defects in Lens

1. Chromatic Aberration

When a parallel beam of white light incident on a thin convex lens, due to prismatic action of lens different colours are focused at different points on the principal axis. The violet rays are focused nearer to the lens and red rays are focused farther from the lens . Due to this inability of lens to form different colour of  white light at same position on principal axis, the image  formed is coloured and blurred  . This defect is called chromatic aberration.

Mathematically Chromatic aberration =  

Removal

To remove this defect we use two or more lenses in contact in place of single lens .

2. Spherical Aberration

Inability of lens to form the point image of point object on the axis is called Spherical Aberration.

In this defect all the ray  passing through a lens are not focussed at a single point and the image of a point object on the axis is blurred.

Removal :

A simple method to reduce spherical aberration is to a stop before and in front of lens.

3. Coma

When the point object is placed away from the principal axis and the image is received on a screen perpendicular to the axis , the shape of the image is like a comet. It is due to the linear magnification produced by different zones of lens  is different for a point object situated away from the principle axis.

This defect is called coma .  

Removal

It is reduced by appropriate stops placed at appropriate distance of lens.

4. Distortion

when extended objects are imaged , different portion of the objects are in general at different distances from the axis. The magnification is not the same for all portions of the extended object. As a result a line object is not imaged into a line but into a curve.