Choosing the right spotting scope
A spotting scope is a small portable telescope designed for long distance birdwatching or stargazing, but can be used also for hunting and various other outdoor activities. Depending on the type and design the useful magnification will range typically from 20× to 60×.
Because the magnification of a spotting scope is much greater than that of binoculars, this high power means a narrower field of view, a decreased image brightness, and also depth of field. Other problems at higher magnifications are the distortion and lack of clarity caused by heat waves, haze, and air pollution as well as vibrations.
The light-gathering power and resolution of a spotting scope is determined by the diameter of the objective lens, typically between 50 and 80 mm (2.0 and 3.1 in). The larger the objective, the bigger and more expensive the scope.
Eyepieces are usually interchangeable to give different magnifications, or may consist of a single variable “zoom” eyepiece to give a range of magnifications. Magnifications of less than 20× are unusual, as are magnifications of more than 60× since it can lead to poorer image brightness, a narrow field of view, and show too much image shake, even on a tripod. The eyepiece mount layout can be “straight-through” (the eyepiece is on the same axis as the body of the scope), or “angled” (the eyepiece is at an angle of about 45 degrees to the body of the scope).
The aperture of a spotting scope is the diameter of the objective lens, usually measured in millimeters. For birdwatching purposes, the aperture size will normally range from 50 to 90 mm, and this size is usually directly related to the total size and weight of the scope. The size of the objective lens determines the amount of light that will enter the optical system, and the greater the size the greater the image detail and clarity.
Field of View
The widest circle of the viewing area through a telescope is the field of view. This is normally measured in linear feet at 1000 yards or in angular degrees. Because the field of view normally decreases with increasing magnification, this dimension (from 1 degree or 52.5 feet to 3 degrees or 157 feet) will usually be smaller for a spotting scope than for binoculars (right). Although field of view is most limiting at close distances, this narrow range for spotting scopes is adequate because of the medium to long distance observation for which most spotting scopes are used. The closest distance that you can focus on a bird through a spotting scope is typically 20 to 40 feet. Field of view is largely determined by eyepiece make up. Some eyepieces are designed to present wide fields of view (wide angle), and these are very useful and popular for following a moving bird. Eyepieces designed for long eye relief generally have more narrow fields of view. Zoom eyepieces will usually have a more restricted field than an equivalent eyepiece of fixed focal length.
The eyepiece design is a factor in magnification, field of view, exit pupil size, and eye relief. The optical design and glass quality of the eyepiece can also affect the amount of colour or distortion of the image. As mentioned previously, eyepieces differ in many ways. Some are fixed in focal length; some can change the power over a range of 40X or more (zooms). Others are designed to give either wide fields of view (wide angle) or long eye relief (for eyeglass wearer comfort). They can attach to the scope by different means: screw threads, bayonet mounts or by fastening with a set screw. Some spotting scopes have eyepieces that are non-interchangeable (usually these are either zooms or waterproof scopes).The eyepiece placement may be constructed for straight through, forty five degree, or ninety degree viewing (right). Also, some eyepieces are available in different diameters, varying from 0.96″ to 2″. Eyepiece position is usually a personal preference with the straight through design being preferred by many birders. This design makes it easy to sight an object and follow as it moves and is also convenient to use with a car window mount. A spotting scope with an offset forty five or ninety degree placement is easier to use for viewing above the horizon or at tree top level and does not require a tall tripod. Thus, this configuration is convenient for sharing your observations with other birders who are of different heights. The main drawback to the angled eyepiece is the often considerable practice needed to quickly locate a bird in the scope.
Eye relief is the distance behind the telescope eyepiece where the light rays from the object entering the scope exit through the eyepiece to form a magnified, circular image (exit pupil). The location of this point is where the complete field of view is clearly visible to the observer. This distance is most important for those who wear eyeglasses as the minimum distance for comfortable viewing is about 15mm. Although the user of a spotting scope can accommodate for near or far sightedness, those with mild or severe astigmatism will almost always need to wear their eyeglasses for comfortable viewing. Fortunately, many manufacturers now offer eyepieces with long eye relief or at least provide fold-down rubber eyecups for convenience to those who wear eyeglasses (right).