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Common Terms Associated with Binoculars, Monoculars and Spotting Scopes.
The aim of this section is to briefly explain the most common terms and features that appear in optical instruments descriptions. It is designed for use in conjunction with the Buyers Guide and Recommendations By Use sections.
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Binoculars usually have a set of numbers displayed on the body shell that give you information about the binocular specification e.g. an 8x42 binocular has a magnification of 8x and an objective (front) lens size of 42mm. The majority of binoculars will have a set of two numbers to describe the specification. Models with 3 numbers have variable magnification – Zoom Binoculars e.g. 8-20x50 has a range of magnification from 8x up to 20x and an objective lens of 50mm.
It is also common for the Field of View to be displayed on the binocular.
This is the power or strength of the optics. The simplest way to describe magnification is how many time closer the viewed object appears e.g. A binocular with 10x magnification will make an object that is 50m away appear to be 5m away.
Higher magnification does not necessarily mean a better binocular. The best magnification depends on the intended use and personal preference (how comfortable the level of magnification is for the user). See the Recommendations By Use section.
As magnification increases, field of view and brightness decrease (assuming the style and quality of the binocular is unchanged). Images will therefore appear darker and loose some of the colour contrast. Higher power also reduces the stability of the image as it magnifies the users movements (breathing, heart beat, hand shake).
Objective (Front) Lens Size
This is usually the final number of the specification found on the instrument; it is the diameter of the lens expressed in millimetres e.g. 8x42, 10x50. This measurement is the most important factor in terms of image brightness as the size of the front lens governs the amount of light entering the binoculars. It will to some degree determine the overall size of the binocular (see body style).
Small increases in lens diameter can have a significant affect on light gathering capabilities as the lens area is increased exponentially with larger diameters. Large diameter lenses are particularly useful for low light, night sky viewing and high power observation models.
The style, shape and size are influenced by the arrangement of the prisms. There are two designs known as Porro prism and Roof prism.
The basic porro prism design is easily recognisable as the ‘traditional’ style of binocular with an offset barrel appearance. This design is widely considered to produce better optical performance when compared to roof prism designs as the light / prism interaction is simplified.
Standard Porro prism binoculars are larger than their roof prism alternative but compact porro prism designs are available – the prism layout is angled or reversed so the objective lenses can be closer together reducing the body shell size.
The standard porro prism design will give a greater level of depth perception due to the objective lenses being further apart.
Roof or DCF prism designs are the straight-barrelled style of binoculars. DCF models are usually more compact than standard porro prism designs but this means brightness is reduced for the same quality / lens size.
The design of roof prism binoculars is more suited to the manufacture of optics with additional features such as waterproofing and long eye relief. These features are therefore likely to be present on more affordable models than the porro prism alternative.
Field of View
The field of view can be simply described as the width or size of the area that can be seen when using an optical instrument. This is often displayed on the body shell of a binocular in the form of a linear or angular measurement.
The linear field of view is the distance across that can be viewed at a fixed distance away from the user e.g. 84m @ 1000m.
The angular field of view is expressed in degrees e.g. 5.3°.
In order to evaluate binoculars where the field of view is expressed in different formats a good approximation is 1° roughly equals 17.45m (@ 1000m) – not exact but useful for quick comparison.
As a general rule in a given range of binoculars the field of view will decrease as magnification is increased. The actual field of view is determined more by the prism layout than magnification.
A larger field of view is beneficial when viewing moving objects.
Prism Qualities and Prism/Lens Coatings
The biggest factor in prism quality is the grade of optical glass used. There are two main grades for binocular prisms BAK4 and BAK7; the overall image quality produced by a prism is also affect by application of coatings.
Of the two grades BAK4 prisms are the superior quality, they are manufactured from a higher density glass resulting in higher resolution images and enabling more efficient light transmission i.e. they produce sharper and brighter images.
There are several stages/levels of prism and lens coatings aimed a maximising images resolution, brightness and light transmission:
Coated optics – one or more surface is coated
Fully coated – all air to glass surfaces are coated (prisms and lenses)
Multi coated – one or more surface is multi coated (multiple layers of coating)
Fully multi coated – all air to glass surfaces have multiple layers of coating
Anti-UV coatings – The lens coatings are used to improve the sharpness of an image in bright or hazy conditions. They reduce unwanted reflections and help to sharpen the image. The most commonly seen anti-uv coating is a ruby red colour but gold, green and blue have been used.
The Anti-UV coating gives the lens a more reflective appearance than other forms of lens coating (when viewing the binocular lenses not using it). Some sellers’ claim that binoculars with ruby anti-uv lens are night vision models – this is not correct and in reality the anti-uv coating is more likely to reduce the binoculars ability to be used in low light conditions.
It is also worth noting that the red ruby anti-uv coating is too harsh for bird watching as it gives the image a blue hue affecting the ability to identify colours and markings on birds.
Phase Coatings (DCF models only) -