GUIDE TO BINOCULARS
The following information from Opticron aims to give you a basic understanding of how binoculars work, the different specifications available and what they mean, plus some points to help you choose the best instrument suited to you.
Specification Example: 8×42. The number ‘8’ denotes the magnification and means an object appears to be 1/8th of its actual distance away. Using this rule an object 40m distant appears 5m away. ‘42’ is the diameter of the objective lens (the large end) in mm through which light enters the binocular.
Z or B denote porro prism which usually offer better optical performance for your money compared to roof prism D, which are more compact.
The popularity of roof prism binoculars is a result of this system being favoured in the development of instruments with user oriented features such as waterproofing, long eye relief and close focus.
B denoted on roof prism binoculars including Opticron BGA = full field of view for spectacle wearers. GA = rubber armouring and W or WA = wide angle. Centre focus CF, is more popular than individual eyepiece focusing IF especially when you need to focus at different distances under 50m. Most IF binoculars are 7x magnification and have a large depth of focus which usually eliminates the need for focusing adjustment at distances over 50m.
What magnification? The higher the magnification relative to the objective lens size, the lower the brightness and the shallower the depth of focus (distance in focus at a single focus setting.) For general observation choose 7x or 8x. If you want 10x or more try them first as magnification amplifies hand-shake affecting image stability.
What objective lens size? The amount of light entering a binocular is related to the surface area of the objective lens ‘OG’. A 50mm OG will admit 2.5x the light of a 30mm OG. The amount of light reaching the eye is called the exit pupil diameter ‘EPD’ and its size can be found by dividing the OG diameter by the magnification. For example the EPD of an 8×32 = 4mm while the EPD of an 8×56 = 7mm. As a general rule your iris dilates 2-3mm in bright sunlight, up to 6-7mm in twilight. With an EPD of 5-6mm 7/8×42’s and 10×50’s will outperform 8×32’s and 10×42’s but are larger and heavier for their given magnification.
The field of view of a binocular is dependent on the optical design and is expressed as either the width of panoramic view in metres from a distance of 1000m or in degrees where 1° is approximately 17.45m.
Wearing glasses Many binoculars provide the full field of view when wearing glasses by either turning/pushing the retractable eyecups to the ‘down’ position or folding down the rubber eyecups. Binoculars with a stated eyerelief of 15mm or more deliver the full field of view for most spectacle wearers.
Weight For most people, small, lightweight binoculars will be used much more than large heavy ones which tend to be left at home or in the car.
Prismatic telescopes are often called spottingscopes or fieldscopes and follow the basic design of a large monocular. Opticron fieldscopes come in ‘body only’ format allowing you to choose an eyepiece or eyepieces to suit your hobby and budget.
Eyepieces are described according to their magnification when fitted to a particular spottingscope and whether they are wide-angle e.g. 20xWW or zoom e.g. 20-60x. A 20x eyepiece makes an object appear 1/20th its actual distance compared to your naked eye. Using this rule an object 500m distant appears 25m away at 20x, 16.7m at 30x and 8m at 60x.
What magnification/objective lens size? Spottingscopes are most often used for high magnification daylight observation at distance. In normal daylight when your pupil is dilated 2-3mm, a 66mm fieldscope will deliver optimum performance, (the balance between magnification and image brightness) between 22x and 35x, i.e. when the exit pupil diameter of the fieldscope equals that of your iris. In low light when your pupil dilates 5-7mm, optimum performance can only be achieved by lowering the magnification or increasing the size of the objective lens.
The higher the magnification, the greater amount of image and colour distortion. These effects can be reduced by using ED or Fluorite lenses in the objective system but run at a premium over standard optical glass lenses.
Field of view is usually expressed as the width in metres of the image when viewing at a distance of 1000m and is directly related to the magnification. The higher the magnification, the smaller the field of view. The objective lens, irrespective of diameter has no influence over the field of view.
Light transmission One way to assess the brightness of a fieldscope and eyepiece is to calculate the exit pupil diameter in the same way as with a binocular and make a trade-off between brightness and magnification desired. For general daytime terrestrial observation good compromise magnifications are 18-25x(50mm), 20-30x(60mm), 25-35x(66mm) and 25-40x(80mm).
Resolution As a general rule a good spottingscope can resolve (separate) two black dots 1.5mm distance apart on a white surface, in bright daylight at a distance of 50m.
Which Eyepiece? The vast majority of spottingscopes are used with zoom eyepieces as they offer the greatest flexibility for viewing at different distances in different light conditions. We also offer you a wide range of eyepieces for specialised applications and there is no substitute for side by side testing if you have the opportunity to do so.
Wearing Glasses Eyerelief is the distance between the eye lens and the point where your pupil is positioned to obtain full field of view and varies slightly from eyepiece to eyepiece. In some cases the eyerelief may be shorter than you need to obtain the full field of view. Almost all Opticron eyepieces offer the full field of view for most people who choose to use them with their glasses on.