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.
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