What is an Achromatic Lens?
An Achromatic Lens is the one which is so designed to completely control the effects of chromatic distortion or even aberration. It is made by the combination of lenses of two different types with different focal powers so that the light from both together are free from achromatism.
It is also known as achromat. The lenses are corrected by making the wavelengths (which are mostly red and blue in colour) of both the lenses to converge on the same plane.
Doublets are the most widely used achromatic lens. It is made of a pair of glasses one of which is concave (made of flint glass) and the other is convex (made of crown glass). Other examples of achromatic lenses are Clark doublet, Steinheil doublet, Littrow doublet etc.