Choosing Your...

Spotting Scope

Wondering how to buy a spotting scope? Choosing a spotting scope can be a daunting task. Optics by Mail have been advising customers for years on the best spotting scopes for their own personal needs.

Most spotting scopes work just like refractor telescopes. The majority of spotting scopes, like 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 eyepieces.

Spotting Scope specifications

For example 15x50, the number "15" represents the magnification power which in this case means it brings objects 15x nearer when compared to the naked eye. The second number "50" represents the diameter of the objective (OG) lens in mm (millimetres) through which light enters the spotting scope. A 15~45x50 has a 50 OG (objective glass) with a zoom eyepiece that enables a variable magnification between 15x and 45x. Many manufacturers now offer spotting scopes in a body only format, which allows the customer to individually select the most suitable eyepiece for their requirements.

What spotting scope magnification should I get?

Spotting scopes, like terrestrial telescopes, are most commonly used to provide high magnification viewing over long distances. Image quality at different magnifications will largely depend on the design of the optical system, the quality of glass used and the coatings applied to the surfaces of each lens. However there are a few general rules that can be applied in determining the right specification for your needs.

Firstly, the relationship between the magnification, OG lens 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 approximately 2.5mm, a 60mm spotting scope will deliver optimum performance at 24x magnification i.e. when the exit pupil diameter equals that of the iris. In twilight when the pupil opens to approximately 7mm the same spotting scope will deliver optimum performance at just 9x. As with binoculars, the higher the magnification, the shorter the depth of field and the narrower the field of view. As magnification is increased it also increases image and colour distortion. This effect can be reduced by using ED or Fluorite combination objective lenses that minimise chromatic aberration, enhancing overall contrast and clarity across the magnification range. At magnifications of 10x or lower, the benefits of these expensive lenses are hardly noticeable when compared to conventional glass objectives.

Light transmission

The relative light transmitting capacity of a spotting scope (Twilight Factor) is sometimes said to be found by calculating the square root of the multiplication of the objective lens diameter. Using this calculation to assess image brightness can be confusing as under these rules a 20x70 combination will give a lower light transmission than a 30x70 combination. The best way to assess the actual brightness of any spotting scope and eyepiece combination, assuming equal optical systems, is to calculate the exit pupil diameter in the same way as with a binocular. See choosing a binocular.

Field of view

As with light transmission, the field of view is directly related to the magnification. The greater the magnification, the smaller the field of view. The field of view is normally expressed as the width in metres of the image when viewing at a distance of 1000m. There are exceptions, namely wide-angle eyepieces designed specifically to offer greater fields of vision. It is important to note that the objective lens, irrespective of diameter has no influence over the field of view.

Resolution

As a general rule a good spotting scope should be able to resolve two black dots 1.5mm distance apart on a white surface, in bright daylight from a distance of 50m.

Eye-relief

Eye-relief is the distance between the eye lens and the point where the pupil is positioned for full field of view. This varies from eyepiece to eyepiece. In most cases the eye-relief is shorter than that required by spectacle wearers to obtain the full field of view - especially at higher magnifications.

Straight-through or 45° angled

Common advantages of an angled spotting scope are that the human upper body is in a more relaxed position when looking through the scope and the tripod can be set at a lower position making it easier for people of different heights to use. This is therefore likely to mean the equipment will be more stable in outdoor conditions.

Straight-through spotting scopes are easier and more natural to use when following fast moving objects, using the instrument from the confined spaces of a hide or vehicle, or when hand-held.

Tele-photography

Many spotting scopes can be converted into long focal length telephoto lenses for wildlife and general photography. It should be noted however, that due to the small 'f' numbers and comparatively long exposure times needed, high-rated ISO/ASA films and a solid support should be used when attempting this.

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