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 abberration.

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 mismatch index.

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 aberrations are a function of the dispersion characteristics of optical material.
There are two forms of Chromatic Aberration:
• Longitudinal Aberration which result in different focal points for different wavelengths.
• Transverse Aberration which cause different magnifications for different wavelengths.

A collimating lens system is designed to image a point light source in such a manner that al the emerging rays are parallel to each other.

Collimation is a general term for the imaging of a focal point to infinity.

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 is a general term used for difference in brightness or colour. There are two types of Contrast:
• Photometric Contrast which is the difference in brightness between two luminous surfaces
• Physiological Contrast which is the difference in brightness or colour as seen by the eye.
NOTA: Contrast in the context of Transfer Function is termed Modulation o MTF.

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

Distortion is a lens system aberration characterised by 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 centres
• Pin Cushion Distortion where off-axis straight lines are imaged curving away from the centres.

The distance from the principal point to the focal point. (see Focal Length)

F-Number is the ratio of the focal length to the entrance pupil diameter of an imaging system.

The field of view is the outer most point of the field angle capable of being transmitted through a lens system to form an image.

A field stop, an aperture stop, or images in either object or image space can induce this spatial limitation.

Focal Length is defined as the distance between the principal plane and the corresponding focal point for parallel rays.

Focal Length is a function of lens radii, glass type and thickness and is the most important characteristic of any optical imaging system.

The Index of Refraction is the ratio of velocity of light in a vacuum to the velocity of light in a optical material at a certain wavelength:
• N=Cv/Cm

Magnification is the ratio of image size to object size measured normal to the optical axis:
• m'=y'/y

MTF is a quantitative description of the image forming power of an imaging system.

To measure MTF, increasingly fine lines of known contrast are imaged by the optical system and the image modulation is measured in the image plane.

The ratio of the image modulation to the object modulation for different line resolution and separation (spatial frequency) yields the modulation factor. The MTF is a plot of this factor versus spatial frequency.

Most MTF's are calcualted.

Numerical Aperture is defined as the sin of the half angle of the widest ray bundle capable of entering a lens, multiplied by the index of refraction of the medium through which the ray bundle passes.
• NA=nxSin u

The term Numerical Aperture has special significance for microscope lenses.

An optical system able to focus an image at infinity.

In optics, the term Optical Axis has two meanings: the symmetrical axis ofoptical imaging systems and the direction where no birefringence occurs in optically birefringent non cubic crystals.

Optical Density (OD) is the logarithm, base 10, of the ratio of the incident light to the transmitted light:
• OD=log10 Io/I

where Io is the incident intensity and I is the transmitted intensity. If neutral density filters are placed in series, the optical density of the combination is the sum the individual density values.

Optical Glasses are transparent, usually amorphous, and essentially homogeneous materials whose manufacturing processes are controlled in such a way as to create a desired variation in glass material characteristics such as Refractive Index, Transmission Range, Dispersion, etc.

The OPL of a light ray passing through a medium of constant Refractive Index, is the product of the geometrical distance d and the index of refraction n:
• OPL=nd

In a lens system, that surface at which the projections of an entering and exiting ray intersect.

In optics, Reflection is the return of radiation upon contact with a boundary between two different media.

There are two types of Reflection:
• Diffuse Reflection from a rough surface
• Direct Reflection from a smooth surface.

The characteristics of Reflection at the boundary of a weakly or non-absorbing media are summarized by Fresnel's equation.

An optical system able to transfer an image plane to another with appropriate Magnification.

Is the measure of the ability of an optical component or instrument to image two closely adjacent object details as two separate details.

In general Resolving Power is given as the angular distance at which these details appear or as the number of resolvable lines per mm.

Is an optical system where the aperture stop is located at the focal point of the objective lens resulting in the chief rays being parallel to the optical axis, i.e. the exit pupils at infinity.

f light is incident on a boundary between two optical media of different optical densities and is incident from the denser medium, it will experience Total Internal Reflection.

The critical angle for the two materials is described by Snell's law:
• sin ac=n'/n

where n' is the index of the denser medium.

Is the passage without frequency change of radiation through an optical medium.

Vignetting is a mechanical limitation of oblique light rays passing through an optical system.

Vignetting cannot be caused by the aperture stop.

It is radiation which as the capacity to generate visual sensation.

Spectral Range of Visible Light lies between 380nm and 780nm.

It is the distance between the first lens and the object plane.

It is an optical system of variable Focal Lenght, with the Focal Plane remained in a fixed position, while some of the elements move along the lens axis.