Binocular Buying Guide

8/08/2017 11:39 AM

Binocular types, features and sizes vary greatly based on their purpose, ranging from small, foldable binoculars for the theatre or opera, to high-powered, tripod-mounted binoculars ideal for those interested in astronomy. We explain some basics around different types of binoculars and their common features, and give some tips on what to consider when buying binoculars.

How do binoculars work?

Binoculars incorporate at least two lenses: the objective lens (or distal objective lens assembly), which is closest to the object, and the eyepiece lens (or ocular lens assembly), which is closest to the human eye. Generally combining convex and concave lenses, the objective lens forms a clear image that is magnified by the eyepiece lens.

The most common kind of binoculars are prism binoculars. Using convex lenses for both objective and eyepiece lens, a prism system is incorporated in the optical path to rectify the inverted image projected by the objective lens. The use of a prism system in binoculars as illustrated below allows for a much more compact body compared to the Galilean binoculars that don't incorporate prisms.

porro, roof galilean binoculars

Binocular Types

Porro prism binoculars:
In general, there are two types of prism binoculars: porro prism binoculars and roof prism binoculars. The optical path in porro prism binoculars is bent like the letter Z, which means the image formed by the objective lens is reflected by two prisms. The design of this prism system gives the binocular its distinctive offset shape: the eyepiece lens and objective lens are not aligned. Due to the completely reflective surfaces in the prism assembly, no light is lost on its way from the objective lens to the ocular lens assembly. Porro prism binoculars are easy to produce, which may be a reason why they are more commonly used than the more expensive roof prism binoculars.

Roof prism binoculars:
As opposed to a porro prism binocular, the ocular and objective lenses in a roof prism binocular are lined up in a straight line. As the area where the two prism blocks meet must be light-reflective at points and allow light through at others, this design makes it harder to accomplish a crisp, sharp imageiii. Due to their more ergonomic design and usually higher impact resistance than porro prism systems , and because many models feature an internal focus system, roof prism binoculars may be more expensive, but have a huge and loyal fan base.

Compact / foldable binoculars:
If there's no need for a standard, full-size binocular, compact variants can be a valid alternative. Foldable models are usually very compact and lightweight. They can easily fit into a bag, and are ideal for concerts, theatre, or the opera. Their magnification power shouldn't be larger than 5x.

Wide angle binoculars:
The angle of view (AOV) is the angle expressed in degrees between the left and right extremes of the field of view (FOV, see below) and the center of the objective lenses of your binocular. Wide angle binoculars give you a wider field of view, ideal for spotting objects in larger areas.

Zoom binoculars:
As opposed to "fixed-power" binoculars, zoom binoculars have variable magnification, and are described by their magnification range (e.g. 10-30x60 or 12-36x70).

Waterproof binoculars:
Binoculars that are water and shock proof are the perfect solution for any adventures that can get wet and rough, for example on a speed boat, on a bumpy safari, or any outdoor situations with a higher risk of rain. Be sure to check manufacturer's specifications to find out about the level of waterproofing. Some models are just water resistant, i.e. protected from water splashes, while others are fully sealed and can be completely submerged.

Focus-free binoculars:
Sometimes misleadingly called self-focussing or auto-focussing binoculars, focus-free binoculars don't auto or self focus at all, as they have no focusing mechanism. Instead, they are equipped with a fixed depth of field from a close distance to infinity, and everything within this range will remain in focus.

High-powered binoculars:
Ideal for long-distance terrestrial viewing, high-powered binoculars have a particularly high magnification. This also makes them popular among astronomers, who can use high-powered binoculars as an alternative to a telescope.

Tripod mounted binoculars:
Similar to cameras with a large zoom range, binoculars with a very high magnification power will deliver a clearer, steadier view when mounted on a tripod, as this eliminates the shaking and trembling that happens during handheld observation.

Night vision binoculars:
Put very simply, night vision devices transform incoming photons into pools of light visible to the human eye, creating an image even in the complete dark. Night vision binoculars are used for recreational activities, as well as sport or surveillance purposes .

Magnification and brightness

The numbers that describe binoculars (for example the Leica Binocular Trinovid 8X20 BCA) relate to their magnification power (e.g. 8x, 10x, 12x etc.) and the diameter of their objective lenses (e.g. 50mm, 42mm, 36mm etc.). The greater the power of their magnification, the larger the binoculars will make your viewing objects appear as you look through them. For example, if your viewing subject is 150 metres away, and your binoculars have a magnification power of 10x, the subject will look the same size as a subject that is located only 15 metres away when not using binoculars. A larger diameter of the objective lens allows for more brightness and detail, as it can admit more light than smaller diameters

Digital magnification, often comes with zoom functionality and electronic image stabilization. Some digital binoculars, for example from Sony, even allow you to record videos and take pictures . These binocular variants are also referred to as 'Binocams'.

Interpupillary distance, diopter adjustment and eye relief

Besides adjusting the distance of the eyepieces of the telescope lenses in order to match the interpupillary distance of the user's eyes, diopter adjustment is available in binoculars as well. This means that you can adjust the lenses according to the visual acuity of your left and right eye individually in order to get a crisp, sharp image .

Another component that can affect clarity and viewing comfort is 'eye relief' - the distance from the outer surface of the eyepiece lens to the eyepoint. "Looking through binoculars from the eyepoint, you can obtain the whole field of view without vignetting. It is recommended for eyeglass wearers to use binoculars with a longer eye relief (high eyepoint)" , so they're not restricted to a smaller field of view.

Field of view (FOV) and range finding

Your binoculars' field of view is the width of the scenery able to be observed. The FOV is usually described in two ways: 'angular' or 'real' field of view, and 'linear' or 'apparent' field of view. Real field of view is the true angle of the visible field seen through the optics of your binocular. It's measured in degrees, from the central point of the objective lens. The larger the value is, the wider the FOV. A greater magnification yields a smaller field of view. In general, a smaller magnification with a larger FOV is great for scanning scenery, and is ideal for discovering and observing fast movements .

The linear field of view on the other hand describes the angle of the magnified field seen through your binoculars. "The larger the apparent field of view is, the wider the field of view you can see even at high magnifications"viii.

Many binoculars are also equipped with a range finder. A scale superimposed on the view lets you estimate your distance to the object if the object's height is known. This feature can come in handy while hunting or golfing.

Optical quality and lens types

Clarity is the main component attributed to optical quality. High-quality lenses available with several coatings to optimise the visible light spectrum and eliminate glare are an important factor to achieve superior optical quality, high performance in all conditions, and true colors . Integrated lens corrections in high-quality optics help offset imperfections such as chromatic aberration (color fringing along high contrast edges) or linear distortion .

Image stabilization This self-steadying feature (for example found in Canon binoculars) autocorrects for jitters and shakes that easily occur during high magnification observation - especially when you hold your binocular in your hands, or you are in a moving vehicle. In general, binoculars may be equipped with either a mechanical or digital image stabilizer.

What binocular for what situation?

The type of binocular you should choose greatly depends on its use. No matter if you need a binocular for bird watching, hunting, travelling or the theatre, the list below gives you an overview of the features to look out for.

Application Magnification Field of View Lens size Useful features
General use
Sports viewing, travel, concerts and so on.Best binoculars for general use
8x or 10x Wide FOV 42mm
  • Lightweight
  • Midrange magnification
Bird watchingBest binoculars for bird watching 7x, 8x or 10x Wide FOV >30mm
  • Zoom
  • Image stabilisation
Whale watching and seal watching Best binoculars for whale watching and seal watching 8x or 10x Wide FOV 42mm - 45mm
  • Image stabilisation
  • Zoom
  • Digital recording
  • Water resistant/ water proof
Hunting or safari Best binoculars for hunting or safari 8x or 10x Wide FOV >42mm
  • Image stabilization
  • Range finder
  • Night vision
Best binoculars for astronomy
15x - 30x Widest FOV possible in your chosen magnification >70mm
  • Tripod mount
  • Wide angle
Maritime, marine and tactical purposesBest binoculars for maritime, marine and tactical purposes 7x Wide FOV 30mm or 50mm 
  • Water proof
  • Shock proof
  • Image stabilization
  • Rangefinder
  • Night vision
Theatre or operaBest binoculars for theatre and opera 3x or 5x Wide FOV so you can see the entire width of the stage 25mm - 27mm
  • Compact
  • lightweight


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