CLICK HERE for a Guide to Telescopes
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 E.g. 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 to be only 5m away. ‘42’ is the diameter of the objective lens (the large end) in mm through which light enters the binocular.
Z or B stand for porro prism binoculars. The basic optical design of these ‘traditional-shape’ instruments mean they tend to deliver better optical performance for your money compared to roof prism (D) binoculars, which are more compact but also more expensive.
The rise in popularity of roof prism binoculars is as a direct result of this system being favoured in the development of instruments with user oriented features. These include; internal focusing for waterproofing, long eyerelief eyepieces that enable the viewer to obtain the full field of view with or without glasses and close focus making them useful for watching butterflies and other insects.
B when seen on roof prism binoculars including Opticron BGA = full field of view for spectacle wearers. GA = rubber armouring and W or WA = wide angle vision. CF, (centre focusing) is more popular than IF (individual eyepiece focusing) 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? Within any given range of binoculars the higher the magnification relative to the objective lens diameter, the lower the apparent image quality. Images appear less sharp and have a lower ‘relative’ brightness and poorer colour contrast. Higher magnification binoculars also;
a. have a shallower depth of focus (distance in focus at any one focus setting), requiring more use of the focus wheel
b. amplify natural hand-shake affecting the stability of the image being viewed.
For general observation choose a 7x or 8x magnification. If you want binoculars of 10x or over try them first.
What objective lens size? The amount of light entering a binocular is directly 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 known as 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 the iris dilates between 2 and 3 mm (bright sunlight) and 6 and 7 mm (twilight) and so a choice should be made on the type of use the binocular is being put to. For general use choose a 7x42 or 8x42 binocular with an EPD of +5mm.
The field of view of a binocular is dependent on the optical design and not simply a product of the magnification and objective lens diameter. It is expressed as either the width of panoramic view in metres from a distance of 1000 metres or in degrees where 1° is approximately 17.45m.
Wearing glasses Many binoculars provide the full field of view when wearing glasses by either folding down the rubber eyecups or by turning/pushing retractable eyecup assemblies to the ‘down’ position. As a general rule the longer the eyerelief - the distance between the surface of the eyepiece and the point where the pupil is positioned for full field of view - the better the instrument for spectacle wearers. If you think you want to wear glasses while using your binocular, choose one with an eyerelief of 15mm or over.
Weight For many people, small, lightweight binoculars that can be taken anywhere will be used much more than larger, heavier ones which tend to be left at home or in the car!
The majority of terrestrial telescopes are prismatic and as such follow the basic design of a large monocular (half a binocular) usually between 50mm and 100mm in diameter, designed for use with a range of different magnification eyepieces. These prismatic telescopes are often called spottingscopes or fieldscopes.
Specifications Opticron telescopes are generally supplied in ‘body only’ format e.g. MM2 52, HR 66, ES 80. These models have objective lens diameters of 52mm, 66mm and 80mm respectively, no built-in eyepiece and as such allow the choice of eyepiece to be made separately.
Eyepieces are denoted according to their magnification, whether they are wide angle (WW or WA) or variable zoom eg. 20-60x. A 20x eyepiece allows the object being viewed to appear 1/20th of its actual distance away when compared to the naked eye. Using this rule an object 500m distant appears to be only 25m away. A 30x eyepiece makes the object appear 16.7m distant while a zoom eyepiece set at 60x means the same object appears to be just over 8m away.
What magnification? Terrestrial telescopes are most commonly used for high magnification viewing over long distances in daylight. Image quality at different magnifications will depend on the optical system, the quality of glass used and the coatings applied to the surfaces of each lens. There are however a few general rules that can be applied in determining the right specification for your needs. Firstly, the relationship between magnification, objective lens dia. and the size of the eye pupil, and secondly the quality of the optics inside the ‘scope.
In normal daylight, when the pupil is dilated to between 2 and 3mm, a 66 mm telescope will deliver optimum performance, (the balance between magnification and image brightness) between 22x and 35x magnification i.e. when the exit pupil diameter equals that of the iris. In low light when the pupil becomes larger, dilating to between 5 and 7mm depending on age, optimum performance can only be obtained by using a lower magnification eyepiece or using a larger objective lens telescope such as an 80mm or 100mm. The higher the magnification, the greater the image and colour distortion. These effects can be dramatically reduced by using ED or Fluorite combination objective lenses that minimise chromatic aberration, but are expensive. At magnifications of 30x or lower, the benefits of these lenses are lower in comparison to conventional glass objectives.
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. Generally the greater the magnification, the smaller the field of view. There are exceptions, namely wide-angle eyepieces which are designed specifically to provide greater fields of view. CLICK HERE for more information on Opticron eyepieces. It is important to note that the objective lens, irrespective of diameter has no influence over the field of view.
Light transmission The best way to assess the actual brightness of any telescope and eyepiece combination when choosing for daytime terrestrial use, (assuming equal optical systems) is to calculate the exit pupil diameter in the same way as with a binocular and making a trade-off between image brightness and magnification desired. For general daytime terrestrial observations good compromise magnifications are between 20x and 30x for a 60mm, 25 to 35x for a 66mm and 25 to 40x for an 80mm.
Resolution As a general rule a good telescope should be able to resolve two black dots 1.5mm distance apart on a white surface, in bright daylight from a distance of 50m.
Eyerelief This is the distance between the eye lens and the point where the pupil is positioned for full field of view and varies from eyepiece to eyepiece. In some cases the eyerelief is shorter than that required by spectacle wearers to obtain the full field of view, especially at higher magnifications. If it is important that you obtain the full field of view with spectacles please choose any of the following eyepieces: SDL, HDF (all models), HR; 40930, 40931, 40933, IS; 40916, 40918, HR.MM2 40909. Remember, rubber eyecups although fitted to many eyepieces are no guarantee that the full field of view is obtainable when set in the ‘down’ position and used with glasses.
Straight-through or 45° angled Common advantages of an angled telescope are that: i) the back, shoulders and neck are in a more relaxed position when looking through the ‘scope, ii) the tripod can be set at a lower position making it easier for people of different heights to use and the equipment more stable in outdoor conditions. Straight-through telescopes are easier to use when following fast moving objects, using the instrument from the confined spaces of a hide or vehicle, or when hand-held.
Our advice Compare different models side-by-side at the same magnification if possible. Product reports are generally subjective and are no substitute for individual testing. If you are unable to test before you make a purchase, contact us for information and advice on the best model to suit your needs.
Lastly, suitable tripods for any given telescope will, on average, be around the same weight as the ‘scope. Therefore if you are thinking of walking any distance with your chosen telescope, pick it up together with the tripod you are intending to support it with and take the combination for a ‘test walk’ to see how far you can comfortably carry it.