WHICH SPOTTING SCOPE IS BEST FOR
There are several factors to consider when choosing a
spotting scope. Listed below are the more important criteria that you may wish
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 scopeís
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 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 ó 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 scopeís 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.
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.
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
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 ų 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
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.
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.
The minimum distance between the eyepiece of the spotting
scope and your eye that still allows you to see the entire field of view.
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.
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.
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
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
canít 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 thatís
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 Celestronís C90 spotting scope (1000mm) at the same