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OPTREP GLOSSARY of Optical Terms

Customers often ask the meaning of optical terms and so the following abridged glossary is offered.  It is difficult to know where to draw the line, short of a book, but the following should prove to be the most commonly used terms.

A - B - C - D - E - F - G - K - L - M - N - O - P - R - S - T - V - Z


Abbe prism.
  An expensive type of roof prism, more compact than the porro type and used in, for example, "straight through" binoculars.  Named after Ernst Abbe, the optical researcher.

Imperfect lenses will give chromatic aberration (splitting of the light) or spherical aberration (curved image) which are the main types but there are also coma, astigmatism and distortion types.

  Up to middle age, the eye has the ability to increase the curvature of the crystalline lens, a process called accommodation, and thereby increase its overall optical power.  Correction outside of this requires spectacles or contact lenses.

Achromatic (or doublet) lens. 
Two lenses (usually cemented together) of different glasses (e.g: Flint and Crown) to cancel out chromatic aberration for two primary spectral colours.

Angle of Convergence. 
The angle formed between a point object being observed and the two eyes, giving depth perception.  The angle increases as the point becomes closer.

Angle of Reflection. 
The angle formed between a reflected ray of light as it leaves a surface and the vertical to the surface at that point.  Equal to the angle of incidence of the ray.

Angle of Refraction. 
The angle between a deviated ray of light that has passed through the boundary between two transparent media and the vertical to the boundary surface at the point of emergence in the second medium.

Substances or coatings deposited on optical surfaces under a high vacuum preventing inefficient reflection of light.  Coatings include Magnesium Fluoride, Silicon Dioxide, Titanium Oxide, etc.

Aperture Stop. 
Usually a hole in a thin metal plate, somewhere near the centre of the optical system.  Called an iris if adjustable (as in cameras).  It lets the most light through without allowing the image to degrade at the edge.

A condition of the eye after the crystalline lens has been removed because of cataract opacities.  Usually possible to insert an intraocular lens into the eye to compensate for lens removal.

Aplanatic lens. 
A lens usually used to shorten the focal length of an achromatic doublet lens.

A lens designed to be colour-corrected for three primary spectral colours.

Aspheric lens. 
Aspheric means non-spherical but, in optics, it relates to a lens which has one or more of its surfaces not spherical but of a different curvature.  More expensive than spherical lenses but gives fewer errors.

If the lens of the eye becomes non-spherical, or astigmatic, the vision can be corrected with a suitable toric prescription lens.

Astronomical telescope. 
A telescope without the optics needed to invert the image the "right" way up since this is unnecessary for astronomy.

The optical axis is an imaginary straight line joining the centres of curvature of a lens with, for example, two curved surfaces (the surfaces each being part of a sphere).  It is a longitudinal axis of symmetry of a lens.


Back Vertex of a Lens
(and front vertex) where the axis meets the lens surface/s. 

Barrel Distortion
  (and pincushion distortion).  Image where the edges curve outwards (image where edges curve inwards).

An appalling abbreviation for binoculars!

Barely acceptable abbreviation for binoculars.

Binocular Fusion. 
Sight with two eyes.  Enhances 3-D realism and aids distance judgement.

Biocular Magnifiers. 
Usually military and of lower power than binoculars, typically for viewing night vision optics.

Bloomed lens. 
An old term for anti-reflection coated lens.


Of the light falling on a lens surface, some 4 to 6% would be lost if the lens was untreated.  The lost light is reflected and plays no part in forming the final image.  By coating a lens or prism surface, this loss is reduced and efficiency is increased.  The coating is a film of various materials (e.g: magnesium fluoride) deposited in a vacuum process.  An electron beam coating method is used to deposit harder coatings such as silicon dioxide.

Chromatic Aberration. 
Because lenses and prisms deviate blue light more than red light, a rainbow effect or colour spread will result unless achromatic combinations of optics are used to correct it.

Coherent Light. 
Light made up of photons that have the same wavelength and are all in phase and thereby very energy efficient.  An example is a laser.

Alignment of the optical axis in an optical system.  In a binocular instrument, alignment of both axes and one with the other to avoid eye strain.

Imperfect optical system can make a star appear like a comet, with a "tail".  This aberration can be reduced by stopping down the lens.

A surface of a lens is concave if it curves inwards.

Condenser Lens. 
A positive lens of short focal length, usually used to direct light into a projector lens or the optical system of an instrument.

A surface of a lens is convex if it curves outwards.

The first lens of the eye's optical system, followed by the aqueous humour and the crystalline lens.

Crown Glass. 
One of the most suitable and common types of glass used in optics and often used in conjunction with flint glass (they are of different refractive indices) to make up achromatic lenses.  Lighter plastic equivalents of crown glass used for spectacle lenses are Perspex acrylic and CR39.

Crystalline Lens. 
The second lens of the eye, behind the cornea.

Cylindrical Lens. 
A lens curved in one plane only or one with a curved bias in one axis.



General term identifying sharpness of detail.

Depth of Field. 
Detail in front of and behind the point of optimum focus that gives a zone of acceptable sharpness. 

Depth of Focus Tolerance (in photography).
How flat the film in the camera must be.

(or diopter).  Unit of vergence, giving the refracting power of a lens as the reciprocal of its focal length, in metres.  Minus indicates diverging and plus indicates converging.

Dioptric Power 
(or focal power).  The higher the dioptric power the higher the magnification and the shorter the focal length (and vice versa).

Barrel distortion and pin cushion distortion are the only types which can occur in a centred lens system (see Barrel Distortion).

Dove Prism. 
Prism using one reflection and two refractions.


ED (or Fluoro-Crown) 

glasses giving very high colour correction.

Instruments using interactions between light and electrical fields (e.g: an image intensifier).

Eye Relief.  
Distance from eyelens outside surface to the top of the eyecup of an instrument, e.g: binocular.  Most comfortable when this distance is between 10 and 20 mm.

(or eyering).  A plastic or rubber ring to help position and steady the eye correctly.  Folded down when spectacles are worn.


Field Flattening. 

Reduction or correction of the curvature of an image by use of a specific lens element (usually negative).

Field Lens. 
Usually a single-element lens at or near the focal plane of another lens (usually an eyelens) to flatten the field.

Field of View. 
The extent of an object which a lens system actually images, usually expressed in degrees (e.g: 5°) or so many metres wide at a 1,000 metres distance.

Field Stop. 
An aperture in a plate or one of the lenses in a lens system to control light rays from off-axis points of extended objects.

Flat Field Lens. 
A highly-corrected lens that virtually eliminates astigmatism and curvature to give an almost flat image.

Fluoride (or Fluorite) lens
uses one or more element of Calcium Fluoride made from synthetic crystals to give very high colour correction.  Expensive.

One of the most suitable and common types of glass used in optics and often used in conjunction with Crown glass to make up achromatic lenses (see Crown).

Focal Length. 
Distance from rear nodal point of a lens to the rear principal focus when lens is focused on infinity.  Reciprocal of focal length is the power of a lens.

Instrument for measuring lenses.  Can be of eyepiece, projection or automatic electronic type.



(Reticule or reticle).  Fine lines (wire or markings on glass) viewed through an optical instrument to facilitate measuring, aligning, sighting, etc.



Instrument for measuring the shape of the eye in order to specify the shape of a contact lens.


Lens Testing Chart. 

A test target with various designs to point up lens defects such as barrel or pin cushion distortion, coma, low resolving power and aberrations.

A simple magnifier  to bring the retinal image of a close object into focus without appreciable magnification.


Magnesium Fluoride. 
A crystalline compound used to make a robust, thin-layer coating on a lens surface to reduce surface reflection and increase efficiency.  Commonly has a purple reflection on the lens.

Magnification Power. 
Ratio of eye's retinal image size, aided by a magnifying instrument, compared to largest retinal image obtained with the unaided eye.

Meniscus Lens. 
Lens used to shorten the focal length of a doublet.

When the unaided eye can only focus on close objects.


Nitrogen Purging. 

Filling an instrument (particularly an outdoor one such as a binocular or riflescope) with zero grade or almost pure nitrogen in order to exclude all moisture and oxygen and so virtually prevent subsequent fogging and oxidation of the optics


Object Lens. 
The front lens, in an instrument, closest to the object being observed.

Object glass, an old term for the object lens.

A hand-held instrument for examining the eye to give an approximation of the power correction needed.


telescope in which the power can be changed but refocusing is needed when this is done.

A change in position of the object, as viewed through an instrument, if the viewing eye is moved.  Parallax correction is especially important for a riflescope.

A set of eyepieces (e.g: astronomical) or objectives (e.g: in a microscope) the focus and image sharpness of which is the same as they are swapped around.

Photosensitive material that darkens as the light passing through it strengthens.  Useful in spectacles.

Porro Prism. 
Most common type of prism ("triangular") which inverts image and reverses the direction of the light.

Usually in middle age the eye cannot focus on near objects such as print and so corrective spectacles are needed.

There are many designs of polished, geometric glass shapes designed to "fold the light up" in order to make an instrument more compact.  Prisms give the designer many possibilities and can also be used to invert the image and give a stereoscopic effect by enabling the objective lenses to be set wider apart than the eyelenses (in a binocular for example).


Instrument for measuring short-radius optical devices such as contact lenses.

Real Image. 
An image as focused, for example, on a camera film or a projector screen (as opposed to a virtual image which we see in a mirror).

Redirection of light by a surface which is highly polished.

Redirection of light by its passing from one medium to another (e.g: from glass to air).

Refractive Index. 
Measure of the refracting power of a medium (e.g: 1.6 for water, 1.46 to 1.95 for optical glass, etc.).

Ability of a lens to resolve fine detail.

Resolving Power. 
The ability (of an instrument, the eye, etc.) to resolve the images of two points.  The closer the points are together, the higher the resolving power required.  Resolving power is measured by the angle subtended between the two points and the eye or instrument.  The eye can resolve two points subtended by an angle of 100 seconds.

The inside, photosensitive surface of the eyeball.  It has an array of detectors sensitive to light and colour.

A hand-held instrument for examining the retina.


Slit Lamp. 
An  instrument for examining the eye.


Toric Lens. 
A prescription lens to correct the eye's vision.


Vertex of Surface

(or the Pole).  The point on the surface of a lens where the optical axis enters the surface.  For a convex surface, for example, it is the highest point on the surface.

Use of a field stop in a position which will cut off the light at the edges to provide a better image.

Virtual Image.  
As seen after refraction by a diverging lens or after reflection in a mirror.  Virtual images are used in instruments such as microscopes, rangefinders, telescopes, etc.

Visual Acuity. 
Measure of visual performance in perceiving detail, ascertained by letter charts.


Zoom lens. 
System in which the power can be varied while the image stays in reasonably sharp focus.

A - B - C - D - E - F - G - K - L - M - N - O - P - R - S - T - V - Z


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