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Spotting Scopes Selection Criteria
Spotting scopes are ideally suited for such activities as long distance birdwatching and nature study, telephotography, scenic observing, long distance microscopy, beginners’ astronomy and surveillance.
As an example binoculars are best for close-up birding, but for distant birds, a spotting scope makes a marked difference. Scopes allow you to distinguish marks on distant objects like a red kite that cannot be seen with binoculars. And at closer ranges, you’ll admire intricate plumage details you never saw before.
Astronomy telescopes are much more powerful, but usually do not gather enough light for observing and are not robust enough for use in the field.
Let us consider important criteria for spotting scope selection:
A spotting scope is a compact telescope designed primarily for terrestrial observation and is used in applications which require magnifications beyond the range of a typical binocular usually with a magnification power between 15x and 250x. Ideally, the objective lens should be at least 60 mm in diameter to provide a bright image.
To change magnification, scopes have interchangeable fixed-length eyepieces or a single zoom eyepiece. With a spotting scope the observer can start with a low power magnification (eyepiece or the lowest setting on a zoom eyepiece e.g. in the 20x to 30x range). Once you’ve spotted the target you can then switch to a higher power.
Eyepieces have a bayonet fixing for a quick change and scopes have good focusing techniques to adjust the target. As an example the best all-round eyepiece for a birding scope is 20x to 30x.
Zoom lenses change magnification power from 20x to as high as 60x with a single, simple adjustment. Like binoculars though, scopes suffer from less light, narrower field of view, and more vibration as magnification increases. The old scopes with a zoom lens showed an the image which was too dark to see much detail as you move toward 60x, and viewing at dawn or dusk or poor light was frustrating.
Currently, good scopes have excellent zoom lenses giving sharpness and clear image indistinguishable from that at low magnification. The 25-50x wide angle eye pieces produce excellent field of view and clear images.
If your budget allows to buy a high quality scope, please do it.
Top spotting scope lenses are made with fluorite-coated, HD (high density), or ED (extra-low dispersion) glass. The difference in brightness and image clarity between these high-quality scopes and those made by the same manufacturers using standard glass is particularly noticeable in low-light viewing conditions and at high power.
A large objective lens with suited magnification, results in good light gathering. The ratio between objective diameter and magnification must be as close as possible to 7. This is the average size of the human pupil in low light viewing conditions. The glass quality and coatings also plays major roles here.
In spotting scopes, there are two methods for focusing. A grip around the scope barrel can be turned for rapid focusing or a knob which is turned and is slower but can be more accurate.
You may not want to use one but serious spotters always have a tripod and reap the many rewards from it. Images will be more stable in outdoor conditions. You will often return to the same spot for observing and a scope supported on a tripod is all set and ready to go when your target arrives. So get a mid-weight tripod with as few leg adjustments as possible and a flip-lock design for a secure mount.
Straight or 45° Angled Scope
Advantages of an angled telescope are that the observer 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 tall and short people.
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. Check that this within the right length if you wear spectacles so you can obtain the full field of view.
Depth of Field
The depth of field is the maximum depth behind the focused object , where the objects in the vicinity of the focused object are still in focus. The depth of field is more on high quality scopes, which then result in objects at more different ranges being in focus at any given time.