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    Telescopes Guide

    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.

    Straight or 45° angled? Straight scopes are easier to use following fast moving objects, using the instrument from the confined spaces of a hide/blind or vehicle, or when hand-holding. Advantages of 45° angled spottingscopes are that your back, shoulders and neck are more relaxed when viewing and your tripod can be set lower for increased stability. Angled scopes allow people of different heights to view simultaneously without having to make any adjustment to the height of the tripod.

    Tripods A suitable tripod will be on average about the same weight as the fieldscope it is supporting. If you are thinking of walking any distance with your equipment make sure to test the complete kit before buying.