Optical Glossary

Achromatic
A lens classed as achromatic has been formulated to be free of chromatic aberration, which causes colour fringing. An achromatic lens, or achromat, achieves identical focus with light rays of two different colours.

Apochromatic
Apochromatic lenses are corrected to bring all three primary colours to the same focus.

Astigmatism
Aberration in a lens or optical system that makes lines oriented in certain directions less sharply focussed than lines running in other directions. Astigmatism also occurs in the human eye. A typical astigmatic effect would be to see the vertical lines on a sheet of graph paper sharply while seeing the horizontal lines less clearly, or vice versa.

Bell
In scope sights with ends flared to accommodate large-diameter ocular or objective lenses, the oversized lens housings are referred to as bells, as in ocular bell or objective bell.

Binocular Collimation
Regulating the two halves of a binocular for …