Abbreviations used to describe Optical Instruments.
Ø Aberrations. An optical system can have a number of different aberrations, which are deviations between the actual image and the image formed by the optics. These include spherical aberration; coma in which the image of a point of light appears as a blurred pear-shape; pin cushion in which straight lines curve inwards in their middle; barrel in which straight lines curve outwards in their middle; astigmatism in which light rays are not focused at a single point; curvature of field; and chromatic aberration* in which the wavelengths of different coloured light rays focus at different points. Aberrations never appear singularly, there is always a mixture of two or more different aberrations.
Ø Aperture. There are two types of aperture in an optical system. Effective aperture is the maximum diameter of a light beam passing through the OG* or front lens. Relative aperture is the ratio of the diameter of the effective aperture of a lens system to its focal length.
Ø Bausch & Lomb. A quality American manufacturer ( but now branded BUSHNELL Jan 07)but also the term used to indicate the one piece American style of binocular in which the front lens is fitted directly into the body and not into a separate cone.
Ø BCF Bausch & Lomb* (or American) style body Centre Focus* The front lens is fitted directly into the body.
Ø BIF as BCF but with Individual Eyepiece Focus.
Ø Chromatic Aberration. This is most likely to be found in less expensive optics. It causes a halo around or along objects especially when viewed against a bright background. Often seen as a blue halo one side and a yellow halo on the other. The OG* (object glass or object lens) should comprise two elements of different glass to overcome this.
Ø Coatings. Lenses and prisms can be coated*; fully coated*; multi-coated* and fully multi-coated*. Thin transparent film or films is/are applied to the surface of the glass equivalent in thickness to the wavelength of light. Used to increase transparency or reduce reflection. When light strikes a glass surface, about 5% of the light reflects, reducing the light passing through the lens, increasing internal reflections and causing flare and ghosting. An uncoated system can lose 40% of the light that enters at the front. A fully multi-coated system can transmit over 99% of the light.
Ø Coated. A single layer on at least one lens.
Ø DCF Roof prism* Centre Focus*. Roof in German is Dach.
Ø Depth of Field. A zone extending in front of and behind the focused distance where the blur is too insignificant to be noticeable and for parctical purposes can be accepted as a sharp image. See also FF* and Dual Focus*
Ø Dioptre Adjustment. Usually one of the eyepieces can be focused independently from the centre focus to compensate for individuals different eyesight in their left and right eyes.
Ø Dual Focus similar to fixed focus* but with two fixed positions, one position for near focus and one for far focus.
Ø Eyecup. The ring right at the end of the binocular into which you place your eye. Can be Rubber or Bakelite or Plastic
Ø Eye Lens. The (usually) small lens that you place next to your eye. See OG*
Ø Eyepiece. Can be confusing. The removable rear lens assemblies of a telescope are called eyepieces; the eye lenses fitted in a binocular can be called eyepieces but the metal surrounds immediately under the eyecups can be called eyepieces. The right surround is called the dioptre* eyepiece.
Ø Eye Relief. This is the distance behind the eye lens* of the binocular that you need to place your own eye so as to see the whole of the available field of view ( FOV * )
Ø Exit Pupil refers to the diameter in mm of the circle of light visible at the eyepiece. The larger, the brighter, but our ability to fully utilise the binocular exit pupil, diminishes with age.
AGE DAY NIGHT
20 4.7 mm 8.0
30 4.3 mm 7.0
40 3.9 6.0
50 3.5 5.0
60 3.1 4.1
70 2.7 3.2
80 2.3 2.5
Myopic or short sighted, and you can add a bit
Hyperopic or long sighted, you have to subtract a bit.
Source – Hebditch May ‘47
Ø Focal Length. The distance from the object to the focus point of the image of that object.
Ø FF Focus Free. The focus is fixed at the hyperfocal distance*. Reasonably near and far objects appear acceptably sharp but if you wear spectacles, you will have to keep them on. See also Depth of Field*
Ø FOV Field of View. Sometimes shown in degrees or in so many feet @ 1000 yards or so many metres @ 1000m. The bigger the number of degrees or feet or metres, the wider area you will see from left to right.
For a list of typical Fields of View click on Fields Of View Table
Ø Fixed Focus. See FF*. A focusing system which has a fixed focus at a single point by utilising the depth of field of a lens so that focusing is not required.
Ø Fully-coated. A single layer on all air-to-glass surfaces.
Ø Fully Multi-coated. Multiple layers on all air-to-glass surfaces.
Ø Ghost Image. An unwanted image formed when strong sunlight strikes the front lens.
Ø Galilean. Binoculars without prisms. Usually only now found in opera glasses and limited to about a maximum of 4.5 magnifications.
Ø Hyperfocal Distance. When a lens is focused at a given distance and the depth behind the focused point becomes infinite, the focusing point is designated as the hyperfocal distance.
In many cases of fixed focus cameras and binoculars, the hyperfocal distance is used as the set distance.
In this case the range between half of the hyperfocal distance and infinity is within the depth of field*.
Ø Individual Eyepiece Focus or IF. Each eyepiece can be turned to provide sharp focus.
Ø LER Long Eye Relief. Eye relief* is the distance your eye needs to be to allow full field of view (FOV*). Long eye relief allows you to wear your spectacles or sunglasses and still see the full field of view.
Ø Multi-coated. Multiple layers on at least one lens.
Ø OG The Object Glass or object lens that is the (usually) big lens that you point at the scene to be viewed.
Ø Optical Axis. This is a straight line passing through the centre of the optical system. The system usually comprises lenses and prisms. Each half of a binocular should have an optical axis which points at the same object at the same time. Any deviation will cause a double image that will cause eyestrain and a poor quality view.
Ø Porro prism. The most common shape of prism and the least expensive to produce. Used in both Zeiss* style and Bausch & Lomb* style of binoculars. The front lens is usually offset from the eyepiece providing a greater depth perception and generally a wider field of view*.
Ø Prism Glass is mostly made from borosilicate (BK-7 ) or barium crown ( BAK-4 ). BAK-4 is the higher quality glass yielding brighter images and high edge sharpness.
Ø Phase Correction Coatings are found in the better roof prism* models. Porro prisms* can bring all the different wavelengths of light to a single point of focus but roof prisms can’t unless they have the additional phase coatings. The result is a very noticeable sharper image.
Ø Resolving Power. One of the values that express optical performance measured in line pairs per millimetre. The number quoted is the most alternate black and white lines in one millimetre that can be clearly separated.
Ø Roof prism. The second most common shape of prism. Allows a slimmer style of binocular. A pentagonal octahedron.
Ø ZCF Zeiss* (or European) style body Centre Focus* The front lens is in a cone that is screwed into the body.
Ø Zeiss A quality German manufacturer but also the term used to indicate the most conventional shape of instrument. See ZCF*
Ø ZIF as ZCF but with Individual eyepiece Focus.
This is not the full list but as far as I have got at present. More to follow but if there are any that you can’t find, just e-mail me and I will add it plus it’s definition if I know it.