Technical Glossary

Chromatic aberration is an image defect, reducing the sharpness of an image and originating from the unequal refraction of different light wave lengths. The aberration originates particularly in the edges of an image in the form of color fringes. 

There are two types of chromatic aberration: longitudinal (typical at long focal lengths) and lateral (typical at short focal lengths).

LD (Low dispersion) glass is used to minimize this mistake.  Particularly the photosensitive sensors of digital SLR cameras are sensitive for chromatic aberration; hence, its minimization is especially important in Di and Di II lenses

The above diagram shows the difference in chromatic aberration between optical glass and LD glass elements

Coma is an aberration resulting from beams of light which lie beyond the optical axis. The beams of light pass through an off-axis point causing the lens to focus at different points. The picture points are scattered, and appear as a comet-shaped blur.

With imperfect optical systems this grouping occurs asymmetrically. Instead of a sharp airy disk, a   picture point with a tail directed toward the edge of the optics occurs. This appearance can be diminished by fading out the edges of the rays.