Guide to Binoculars
The following information 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: 8x42. 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 8x32 = 4mm while the EPD of an 8x56 = 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 7/8x42’s and 10x50’s will outperform 8x32’s and 10x42’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 eye relief 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.