TERMINOLOGY

Aberrations:
Aberrations or lens errors occur during imaging when the rays from the object do not converge properly at the image point. Lens aberrations include: spherical aberrations, coma, astigmatism, distortion and chromatic aberration.

Anti-Reflection Coating:
An anti-reflection coating is a single or multi-layer dielectric coating deposited on the surface of an optical element to reduce reflection due to index mismatch.

Back Focal Length:
Back focal length is the distance from the last surface of a lens or lens system to its image plane. Unlike the effective focal length, the back focal length can be measured directly.

Chromatic Aberration:
Chromatic aberrations are a function of the dispersion characteristics of optical materials. There are two forms of chromatic aberration: longitudinal aberrations which result in different focal points for different wavelengths and transverse aberrations which cause different magnifications for different wavelengths.

Collimating Lens:
A collimating lens system is designed to image a point light source in such a manner that all the emerging rays are parallel to each other. Collimation is a general term for the imaging of a focal point at infinity.

Condenser Lens:
A condensing system is designed to collect light emitted by a light source as efficiently as possible and transfer that light to an object point or plane.

Contrast:
Contrast is a general term used for differences in brightness or color. There are two types of contrast: Photometric contrast which is the difference in brightness between two luminous surfaces and physiological contrast which is the difference in brightness or color as seen by the eye. Note: Contrast in the context of transfer function is termed modulation or MTF.

Depth of Field:
Depth of field is a term used for the distance range that an acceptable focus is obtained.

Distortion:
Distortion is a lens system aberration characterised by the imaging of off-axis straight lines as curved lines. There are two types of distortion: barrel distortion, where off-axis straight lines are imaged curving towards the center and pin-cushion distortion, where off-axis straight lines are imaged curving away from the center.

Effective Focal Length:
The distance from the principal point to the focal point (See focal length).

F-number (F/N):
F-number is the radio of the focal length to the entrance pupil diameter of an imaging system.

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