Abbe number
From The Glossary of Digital Photography
A number used to classify the glass used in making lenses and other items. It represents the dispersion of a glass and therefore of a lens. The Abbe number is represented by the symbol V. A high Abbe number represents low dispersion of the glass. The Abbe number measures the difference in refraction of different wavelengths or colors of light. Abbe numbers are used only for dispersion of visible light. Other methods are used for other wavelengths of light. Typical values of V are in the range of 20 to 80. These numbers are important to photography because they are used to minimize chromatic aberration in the calculation of focal lengths in multiple-element lenses. The Abbe number was defined in about 1873 by Ernest Abbe (1840-1905), a young physicist, when he worked with Carl Zeiss in the design of microscopes. The Abbe number is also known as the constringuence of a transparent material.
