The following information is aimed at giving you a basic understanding of how binoculars work, the different specifications available and what they mean, plus some points to help you make a more informed decision about which model to choose.
SpecificationLet us consider a model with the specification of 8x30. The first figure '8' denotes the magnification and allows the user to see an object eight times closer. The second figure '30' represents the diameter of the objective lens (the large end) in mm, through which light enters the binocular. The symbols or letters that often follow provide important information as to which binocular may suit your requirements most satisfactorily.The letters Z or B usually denote conventional porro prism binocular body styles, which gather light from lenses further apart than one's pupils, giving a stereoscopic effect that counters the foreshortening of perspective caused by magnification. The letters D or H stand for roof prism binoculars which are more compact than porro prism models. Centre focusing 'CF' and individual focusing 'IF' are the two main types for adjustment. With a centre focusing wheel model you have a diopter adjustment on one of the eyepieces, to enable the vision to be adjusted and balanced to suit you specifically. With an 'IF' model, generally, once the binoculars have been adjusted to suit your eyes, no further adjustment is necessary, which is particularly useful at sea or where long range observation is required. The suffix B when used on roof prism binoculars means that the eyepieces have a long eye relief, giving full field of view for spectacle wearers. GA or RA stands for rubber armouring and W or WA denotes binoculars that give wide-field or wide-angle vision. What Magnification?For hand held use, practical trials suggest choosing a binocular with a magnification of about 8x and the larger the diameter of the objective lens, the brighter and clearer will be the resulting vision. This relative brightness will be reduced as the magnification increases, as will the depth of field and field of view. Also, the higher magnifications will result in greater hand-shake or tremor affecting the stability of the binoculars.The objective (OG) lens and light transmissionFor any given specification of binocular the amount of light passing through the instrument (known as light transmission) varies according to the optical design, optical quality and lens coatings used. However, a few general rules can help in your choice. The amount of light entering the binocular is directly related to the surface area of the OG lens. A 50mm OG will admit 2.5x the light of a 30mm OG.The amount of light reaching the eye, known as the exit pupil diameter or EPD, can be found by dividing the OG diameter by the magnification, e.g. the EPD of an 8x32 = 4mm, the EPD of an 8x56 =7mm. As a general rule, the iris in the eye dilates between 2.5mm (bright sunlight) and 7mm (twilight) and therefore a choice should be made on the type of use the binocular is being put to. For general use an 8x40 with an EPD of 5mm would be recommended. Field of viewThe field of view of a binocular is expressed as either the width of panoramic view in metres from a distance of 1000m or in degrees. For example: 1° = 17.45m approx. A binocular is usually stated to be a wide-angle instrument if the angular field in degrees when multiplied by the magnification gives a figure equal or larger to 65. The field of view of a binocular is dependant on the manufacturers design and for a given design directly related to the magnification. You may, therefore, find on occasions that a 10x magnification model gives a similar, equivalent or larger field of view as an 8x magnification model. This is because different eyepiece designs are often used within a single range.Setting up the binocularTo adjust to the spacing of your eyes (IPD), simply rotate the two sides of the binocular around the centre hinge until a single rounded picture is obtained when looking through both eyes. Some roof prism binoculars have two hinges and a single rounded picture is obtained by rotating both hinges simultaneously around the centre focusing plate. To set your binocular - with a centre focusing wheel - for your eyes, turn the centre wheel and focus with your left eye (closing your right eye), on an object. When the object is in sharp focus, close your left eye and open your right eye, adjusting the diopter ring until the image is sharp in the right eye. Opening both eyes should reveal the sharpest picture at all distances. With individual eyepiece focusing 'IF' models the procedure is the same except that you turn the left diopter until an object is sharply in view, then close the left eye, open the right eye and turn the right diopter until that eye sees sharply.Wearing SpectaclesMany binoculars provide the full field of view when using spectacles by either folding down the rubber eyecups fitted to the eyepieces or by turning, pushing or pulling the eyecup assemblies as required. As a general rule, the longer the eye relief (the distance between the eye lens and the point where the pupil is positioned for full field of view) the better the instrument for spectacle wearers. |