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There are several factors to consider when choosing a spotting scope. Listed below are the more important criteria that you may wish to consider.

Diameter Of Objective Lense

A spotting scope is really a telescope that produces an upright image. The major function of a spotting scope, like all telescopes, is to gather incoming light. The larger the diameter of the spotting scope’s objective lens, the more light it gathers, with more light translating into a brighter and better image. As the size of the objective lens increases, greater detail and clarity of image will be afforded.

Magnification (Power)

Magnification is the degree to which the object being viewed is enlarged. Magnification, or power, in a spotting scope is a function of the relationship of two independent optical systems — the optics of the spotting scope itself and the eyepiece being used. To determine magnification, divide the focal length of the spotting scope by the focal length of the eyepiece.

By exchanging an eyepiece of one focal length for another eyepiece with a different focal length, you can increase or decrease the magnification of the spotting scope. For example, a 30mm eyepiece used on a C90 spotting scope with a 1000mm focal length would yield a magnification of 33x. A 9mm eyepiece used on the same C90 spotting scope would yield a power of 110x.

There is, however, a limit to how much you can magnify an image. As a rule of thumb, the maximum magnification is equal to 50 to 60 times the diameter (in inches) of the spotting scope’s objective lens (under ideal conditions). Magnification higher than this will produce a dim and fuzzy image. In most cases, a magnification of 20 to 35x is the most useful and satisfying range to use with spotting scopes under normal daytime conditions. Zoom spotting scopes have a single, built-in eyepiece to give you a range of magnifications without the need to change eyepieces.

Focal Length

The distance, in an optical system, from the objective lens to the point where the instrument is in focus (the focal point). The longer the focal length of the instrument, the larger the image scale it offers.

Near Focus

The distance between the instrument and the nearest object you can focus on, while maintaining a good image and sharp focus, defines the near focus. Short near focus is important for close-up visual and photographic work.

Field Of View

The size of the area, in degrees, that can be seen while looking through the spotting scope is the angular field of view. Linear field of view refers to the width of the area that can be observed at 1,000 yards, and is expressed in feet. A larger field of view translates to a larger area seen through the spotting scope. Field of view is related to magnification, with greater magnification creating a smaller field of view, in general. The angular field of view is calculated by dividing the apparent field of the eyepiece by the magnification being used. (AFE ÷ Magnification = AFV). Once the angular field of view is known, the linear field can be determined by multiplying the angular field by 52.5.

Focal Ratio or Photographic Speed (F/Stop)

The ratio of the focal length of the spotting scope to the diameter (aperture) of its objective lens, in millimeters. Spotting scopes with small f/stop numbers react to incoming light faster than spotting scopes with larger f/stop numbers. So, the smaller the f/stop number, the shorter your photographic exposure time.

Exit Pupil

The diameter, in millimeters, of the beam of light that leaves the eyepiece of the spotting scope is the "exit pupil". To calculate the exit pupil, divide the size of the objective lens, in millimeters, by the magnification of the eyepiece being used. The larger the exit pupil, the brighter the image.

Eye Relief

The minimum distance between the eyepiece of the spotting scope and your eye that still allows you to see the entire field of view.

Coatings

Optical coatings are important, as they determine the throughput transmission of a spotting scope. The better the quality of the lens coatings, the brighter the image will be and the higher the contrast of the image will be. Fully multi-coated lenses are the best quality you can choose.

Portability

For applications such as hiking and hunting, portability is a prime factor to consider. For stationary viewing, a large diameter objective lens becomes of primary importance.

Versatility

Most Celestron spotting scopes can be used three different ways: (1). As a telephoto lens when attached to a 35mm SLR camera, (2). For terrestrial observing , and (3). As a telescope. This is the kind of versatility you want.

Construction

Both mechanical and optical construction are important. Alignment of the optical elements (collimation) is critical for achieving optimal results. All Celestron spotting scopes are laser aligned to ensure comfortable viewing, even during extended viewing sessions.

A long telephoto lens brings you close optically when you can’t get close physically. Distance shrinks between you and your subject, allowing you to capture both far away and close-up images on film.

Most Celestron spotting scopes are designed to be dual purpose visual and photographic instruments. For photographic use, all that’s required is a T-Adapter for the spotting scope and the correct T-ring for your particular 35mm SLR camera. With these two simple, inexpensive accessories the usability of your spotting scope is easily enhanced.

The criteria given to consider in choosing a spotting scope applies equally well when choosing a telephoto lens.

The following pictures demonstrate what a telephoto lens can do. The picture on the left is a standard 50mm photo and the picture on the right was taken with Celestron’s C90 spotting scope (1000mm) at the same distance.