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landscape photography, free top tips guide

Which Binoculars should I buy?

By Brin Best, author of ‘Binoculars and People’

Achromatic lens – a lens is described as achromatic if two widely spaced wavelengths of light are focused into the same image plane. Achromatic lens elements consist of at least two slices of glass that have been cemented together or are separated by a layer of air. Such lenses are used in binoculars to improve optical quality and were first developed in Britain by Chester Hall Moore in about 1729.

Alignment – see collimation.

Apparent field of view – the visible breadth, usually expressed in degrees, as seen through a binocular. It can be approximated by multiplying the real field of view in degrees by the magnification.

Apparent magnification – the phenomenon of higher power binoculars not rendering more detailed images due to binocular shake. For example, although x10 binoculars have an ability to see further, binocular shake can reduce their performance to the same apparent magnification as a x8 or x7 binocular.

Aspherical lens – a lens with a surfaces profile that is neither a portion of a sphere nor of a cylinder. Such lenses are used to reduce optical aberrations.

Barium crown glass – a type of high quality glass used to make binocular prisms. Prisms made with such glass are termed BAK-4.

Binocular – an optical and mechanical instrument which uses two rigid parallel tubes, in combination with lenses and/or prisms, in order to make objects appear closer to the user when viewed with both eyes. Note that the grammatically correct form for the singular of the instrument is ‘a binocular’ not a ‘pair of binoculars’, though the latter is in common usage and has become standard form among laypeople. [ see also monocular]

Binocs’ – a shortened form of the word binocular.

Binos’ – a shortened form of the word binocular.

Binocular (or hand) shake – the slight trembling of the hands when using binoculars, usually of higher magnification, that results in the image appearing jumpy and sometimes out of

Binoculars – the plural of binocular, i.e. one binocular, but two binoculars.

Bins’ – a shortened form of the word binoculars which is especially popular with birdwatchers in the UK. For example a birdwatcher might say, ‘There’s a raptor soaring overhead, can you get your bins on it?’.


Brightness – the ability of a binocular to deliver light to the observer’s eyes.


Central focussing (CF) – a type of focussing arrangement which uses a wheel to control both barrels of a binocular to bring objects of varying distance into sharp focus.


Collimation – a binocular is said to be ‘in collimation’ when both optical barrels point at exactly the same spot, providing the observer with a perfectly spherical view when using the instrument. Binoculars that are ‘out of collimation’ give double images and can induce eye-strain as the brain tries to form a single image. The term ‘alignment’ is used synonymously with collimation.


Colour fringing – see chromatic aberration.


Contrast – the ability of a binocular to differentiate between the dark and light elements of a subject. Binoculars render better images when they have increased contrast.


Chromatic aberration (colour fringing) – the appearance of coloured edges to subjects viewed through a binocular, caused by the way in which light is bent by lenses. Some makers use high definition or extra low dispersion glass, or fluorite crystal to control this effect and improve optical quality.


Dioptre adjuster - a mechanism, incorporated in most central focussing binoculars, that allows for the different visual acuity in an observer’s eyes to be accommodated, thereby bringing subjects into sharp focus in both eyes. Dioptre adjustments are often made by rotating one of the eyepieces.


Extra low dispersion (ED) glass – special glass used in some high-end binoculars in order to reduce chromatic aberration.


Eyepiece focussing – see individual focussing.


Eyepiece lens – see ocular lens.


Eye relief – the distance between the surface of a binocular’s ocular lens and the point at which the whole field of view can be seen clearly. Eye relief is particularly important to spectacle wearers, as they cannot get their eyes as close to the lens surface as other users.


Exit pupil - the circular disk visible on the surface of the eyepieces when a binocular is held about 30 centimetres away from an observer towards a bright light. Its diameter, in millimetres, is worked out by dividing the aperture of the objective lens by the magnification. In general the larger the exit pupil, the brighter the image to be seen through a binocular. [see also relative brightness]


Field glasses – an archaic name for a binocular, most frequently referring to those of Galilean construction.


Field of view – the angle of the field visible through a binocular, as measured from the central point of the objective lenses. The larger the value, the wider the field of view and the easier it is to locate objects. Field of view is sometimes expressed in the number of meters or feet at 1,000 units distance.


Fixed focus binocular – a binocular which has no means of focus adjustment and it set to be sharp from about 10 metres to infinity.


Fluorite – a type of mineral used in very expensive optical devices to improve image quality. Chemically a calcium fluoride, fluorite occurs naturally as crystals, but is also grown in laboratories for use in optics. It suffers less chromatic aberration than standard optical glass and is therefore an ideal, if costly, alternative to standard glass. Because good quality fluorite crystals are hard to find and can be difficult to handle, some manufacturers treat glass with fluorine ions, which give it similar properties to fluorite, but make it chemically more stable and easier to shape.


Galilean binocular – the simplest type of binocular incorporating a concave lens closest to the eye and a convex lens at the opposite end of the instrument. Named after the early pioneer of astronomy with a telescope, Galileo Galilei (1564-1642).


Graticule – a scale engraved on a plate of optical glass which is located at the focal
plane and which allows the distance of objects to be estimated. Known as a recticule or recticle in the USA.

High definition (HD) glass
– special glass used in some high-end binoculars in order to reduce chromatic aberration.

Individual focussing (IF) – a type of focussing arrangement which uses the two eyepieces to bring objects of varying distance into sharp focus. Sometimes called eyepiece focussing.

Image (or field) flattener – a special lens element which results in increased edge-to-edge sharpness in certain models of binocular.

Keplerian binocular - a simple type of binocular which uses a convex lens closest to the eye and another at the opposite end of the instrument. Such binoculars produce an inverted view, which can be corrected by the incorporation of prisms. Named after the German mathematician and astronomer Johannes Kepler (1571-1630) [ See also Galilean binocular]

Lens – a component in a binocular made of high grade optical glass.

Magnesium fluoride – a white crystalline salt and the original ingredient in optical coatings applied to lens and prism surfaces to improve light transmission in binoculars. More recent multi-coatings contain several layers which are of varied chemical composition.

Magnification a number given to a binocular to signify the amount of time closer to the observer objects appear when viewed through an instrument. For example, viewing through a x10 binocular objects that are 1,000 metres away appear as if they were only 100 metres away.

Monocular – one half of a binocular.

Multi-coating – the application of several layers of magnesium fluoride on the lens or prism surfaces of binoculars to improve light transmission. State-of-the-art binoculars can have upwards of 20 such layers on one glass surface.

Nocars’ – (pronounced knockers) a shortened form of the word binoculars which is much less commonly-used than the term bins or binos. Its use, especially by women, brings with it humorous overtones due to the implied anatomical innuendo.

Objective lens – the lens furthest away from the user. Light from subjects hits the objective lens first before entering the body of the binocular and many binocular objective lenses have large diameters to increase light input.

Ocular (eyepiece) lens – the lens closest to the eyes of the user.

Opera glass – a small, low power Galilean binocular designed for use in the theatre.

Optical coating – a special chemical layer (originally of magnesium fluoride) which is applied lens and prism surfaces in order to improve the light transmission of a binocular.

Optical munitions – optical instruments designed especially for military uses. They include binoculars, gun sightings telescopes, rangefinders and submarine periscopes which incorporate precise optical systems and are designed to withstand adverse conditions not encountered by optical devices made for commercial use.

Phase contrast coating – a coating applied to roof prisms to improve image quality.

Porro prism – type of prism developed by the Italian optical pioneer Ignazio Porro. It gave rise to the familiar bent body-shape of binoculars launched by Zeiss in the 1890s and which is still a common form today.

Prism – a glass structure that bends light and is capable of inverting an image in optical instruments. Their invention gave rise to the first prismatic binoculars, which appeared in the later 1800s.

Prism binocular – a type of binocular that uses prisms.

Prism plate – a flat piece of metal which conceals the prism cluster of a binocular. Binoculars usually have both front (closest to the objective lens) and rear (closest to the ocular lens) prism plates. This structure is sometimes simply called a ‘cover plate’.

Resolution – the ability of a binocular to pick out fine detail. Though resolution differences between different models are usually apparent to most users in normal use, they can be tested more objectively using a variety of special charts.Roof prism - a type of prism that results in a more streamlined body shape to a binocular than that possible with a Porro prism model. Roof prism binoculars were pioneered in the late 1890s by the German maker Hensoldt and are now one of the most popular types of binocular.

Waterproof – a binocular is said to be waterproof if it is sealed against the ingress of water. Most waterproof models can be immersed fully in water without any water entering the body.

Wide-angle binocular - a binocular is generally considered to have a wide angle if it has an apparent field of at least 60 degrees, though the Japanese Industrial Standard sets the mark at above 65 degrees.

Zoom binocular – a binocular which uses additional floating lens elements in order to enable the user to vary the magnification of the image. Owing to the difficulty of constructing such a binocular and the need for large objective lenses at high magnifications, zoom binoculars are not usually very effective.


Brin’s book ‘Binoculars and People’ can be bought HERE



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