Spotting Scope Guide
By Joe Izrael
Perhaps the most misunderstood of all sport optics is the spotting scope. Although it has now become a de rigueur tool for shooters, hunters, wildlife observers and birders, laymen are often at a loss about spotting scopes. What are these small telescopes and what do they do? Would binoculars be better? Is there an advantage to spotting scopes? What is the best spotting scope for your individual needs?
Spotting scopes are the terrestrial, daytime observation counterparts of astronomical telescopes. Most often they will be prismatic refracting telescopes. However, catadioptric spotters are also available. Typically, a spotting scope will be light and small enough to be transported, quickly set up and used by a single person on a simple (alt-az) tripod.
In the simplest of terms, spotting scopes are very powerful, high-grade magnifiers. They are more powerful than ordinary binoculars, but more compact than astronomical telescopes. Most are reinforced to withstand the rigors of the trail and many models are waterproof and fog proof.
Magnifying the subject makes it look closer than it really is. This is required in many observation scenarios, such as wildlife that must not be disturbed or cannot be approached, spotting ships at sea from shore, forest fires, inaccessible sites, dangerous animals, distant landmarks and many other situations.
For example, skunks, snakes or beehives from a safe 60 foot distance can be viewed at 20x and seem only three feet away, just about arm's reach. It may be impossible to look squarely at a monastery nestled on a mountain, unless viewed from an elevation facing it. At 20x, it will seem 20 times closer and at 30x it would seem 30 times closer. The same would apply to dangerous animals.
The optical quality of the scope will determine how well it can resolve detail, handle distance and dim light. The mechanical quality will determine its precision and durability. Both are key components of lasting value. The single most important quality of a spotting scope, or any optical system for that matter, is quality. Quality with a capital Q.
Before giving you the techno-lingo, a little enlightenment is necessary. Alas, quality cannot be quantified. Characteristics of quality are often esoteric and difficult to describe, hence not provided in a given product's standard specifications. Factors determining the overall worth of an optical system include the quality of the glasses used, ultra precise lens grinding and polishing (errors are measured in fractions of a wavelength of green light), the glass elements' alignment, tolerances, materials, grinding, polishing, nature and number of lens coatings, the type and structure of the prism, the design and quality of the eyepiece and rigorous quality control.
The optical design of each glass element in a telescope is both difficult and expensive, requiring great experience and expertise. So does selecting, or developing, the best type of glass for a particular application. Very few manufacturers (Leica, for example, is one that does) have a glass laboratory where they can develop their own exotic glasses.
Some of the mechanical factors include the materials and construction of the optical tube, precision manufacturing and assembly, light baffling, purging and fog proofing, interchangeable oculars, focusing mechanism, lens positioning/retaining components, tripod mounting system, eye cups, protective finish and so on. Again, a very high level of quality control is essential and this alone is a costly part of the manufacturing process of which most consumers are not even aware.
More difficult yet is describing quality itself. The ability to resolve detail, provide undistorted images, accurately render color is best gauged by actually looking through the scope, not by tossing around numbers and pretentious scientific terms. Often, advertised features, such as high magnification or objective lens diameter are not, in themselves, indications of a high quality spotting scope. Crisp, clear, high contrast images from edge to edge at all magnifications, precision focusing and smooth, easy to operate controls are hallmarks of quality products.
The manufacturer's reputation, earned over decades in the marketplace, is a valuable clue. Brands such as Leica, Zeiss, Questar, Vixen, Leupold and Nikon have earned and maintained reputations for excellent spotting scopes. As they say, past performance is not a guarantee of future results, but it is certainly an indication. You usually get about what you pay for.
Many advancements in optical technology pertain to the glass itself. Here we will attempt to explain some of the more commonly used terms.
Glass coatings are applied to optical elements to reduce the reflection or scattering of light. A properly coated lens will transmit more light than an uncoated lens. Fully multicoated means all glass-to-air surfaces are entirely coated with multiple anti-reflection coatings. Each coating has a slightly different purpose, affecting different wavelengths of light.
Fluorite (FL) and fluoride glass are often components in High Density (HD) and Extra-Low Dispersion (ED) glass elements. HD lenses focus all wavelengths nearly to the same point when achromatic, or at the very same point if apochromatic (APO). Extra-Low Dispersion (ED) glass helps synchronize all colors, projecting them to the same point.
These exotic glass elements are expensive and sometimes difficult to manufacture, but if used properly they can tremendously impact the scope's ability to resolve fine detail, brightness, contrast and the potential to render color faithfully. They also reduce eye strain, eliminate or greatly reduce chromatic aberration and remain clear and bright under lower light conditions. Prices rise accordingly, but optical quality should not be compromised.
The prism system corrects the image so that what you see is right side up and correct from left to right. (Without it, what you see would be upside down and backward.) It also folds the light path, making the scope smaller and more portable.
Most spotting scopes are built with porro prisms; these are easier to manufacture and generally offer superior performance for a given price. BAK-4 prisms use superior glass and offer better optical performance than BK-7 prisms. Roof prisms are employed when streamlining and size reduction are crucial, as they are smaller, but more expensive to produce and require additional phase alignment coatings for optimum performance.
Clear aperture, which in a properly designed scope is the same as the diameter of the objective lens, determines the scope's light gathering capability and, ultimately, how much light is available to reach the viewer's eye. The bigger the lens, the more light it can transmit, but it becomes heavier and more difficult (and expensive) to manufacture. The largest objective that is practical for the intended use is desirable, particularly if a spotting scope will be used in dim light conditions.
Magnification, also called power, is determined by the prime focal length of the telescope and the focal length of the ocular (eyepiece). For any given prime focal length, the shorter the focal length of the eyepiece, the higher the magnification.
The eyepiece is an often underestimated component and there are many different eyepiece designs with different advantages and disadvantages. Ocular lens systems may have from two to eight (or more) individual lens elements. A minimum of three lens elements are required to focus the three primary colors of light to the same point and, in practice, a fourth element is usually required to "clean up" the errors. Additional elements are often used in the eyepiece to increase the apparent field of view, increase eye relief or provide zoom (variable magnification) capability.
No telescope can perform better than its eyepiece; a quality eyepiece is both imperative and expensive. As with the telescope itself, the mechanical construction and finish of the eyepiece is critical to its performance and value, particularly in the case of interchangeable oculars.
Magnification is increased to decrease the apparent distance between observer and subject. Under favorable viewing conditions, this allows the observer to see better and resolve finer detail. However, as magnification increases, exit pupil, eye relief and consequently the overall clarity and viewing comfort decrease. A premium zoom eyepiece retains its brightness and resolution throughout the entire zoom range. Higher magnification will also magnify every slight movement, hence good stabilization (usually a rigid tripod) is required.
Magnification beyond what can be supported by the brightness and contrast of the scene or the quality and clear aperture of the telescope's optics is called "useless magnification." Cheap scopes are often advertised and promoted on the basis of high magnification that they do not have the optical quality or clear aperture to support. Never buy a spotting scope on the basis of advertised magnification.
Exit pupil is the size of the light beam exiting the ocular. Ideally, it should be the size of the observer's eye pupil for the brightest possible viewing. The exit pupil is the quotient of objective size divided by magnification; hence it varies with magnification. Ensure you get adequate exit pupil even at the highest power.
Twilight factor is a mathematical formula that attempts to illustrate how both the size of the objective lens and the magnification contribute to a telescope's ability to show detail in dim light. While both an 80mm objective at 20x and a 60mm objective at 15x have 4mm exit pupils and same amount of light is reaching observer's eye, the scope with the 80mm clear aperture will have a greater twilight factor and that scope's higher magnification will allow it to resolve more detail (other factors being equal).
Eye relief is the distance from the ocular at which the eye should be positioned for the optimum, full field of view through the scope. Too little eye relief and viewing becomes uncomfortable. Eyeglass wearers, in particular, require long eye relief for viewing. Without eyeglasses, 10mm of eye relief may be acceptable; with eyeglasses, 20mm of eye relief (or more) will probably be required.
Remember, advertising and marketing departments twist and tweak the numbers any way they see fit. Just because scopes boast identical specs, does not mean they are actually identical. A fuzzy or distorted image is useless despite all the optical coatings, large objectives and high magnification claimed by the manufacturer.
All the quality in the world won't turn a spotting scope into a magic wand. Visibility will always depend on the ambient conditions. Remember that we are living at the bottom of an ocean of air with endless currents, eddies and floating particulate matter. Fog, haze, mirage, rain, sandstorms and darkness will always interfere with, if not preclude, viewing. However, a premium optic may mean the difference between resolving an aquatic bird across a hazy marsh and just seeing smudges. Similarly, viewable conditions in twilight will be extended by a better scope.
Remember that expectations must be kept in line with reality. A hummingbird four miles away will not be resolved by any telescope through the earth's atmosphere.
Features and options
Spotting scopes are offered in many variations. The viewing system can be angled (meaning the eyepiece slants upward at a 45-degree angle) or straight through. Angled scopes are often more comfortable in the field, especially when glassing for longer periods or multiple individuals are taking turns viewing. Straight bodies are more streamlined, faster to acquire the subject and handier when the scope is positioned at about eye level, as on a shooting bench.
Focus adjusters come in different types: collar or helical style (surrounding the scope's body at its slimmest point), or knob style, protruding above the prisms. Single focusers can be adequately precise, while dual speed focus systems feature a fast adjuster for coarse focusing and a slower knob for precise, fine-tuning. The precision of a focuser depends both on its design and the precision of its construction. Less expensive scopes often sacrifice in this area to cut costs. Buyer beware.
Eyepieces are not always provided with the scope; "body only" or "requires eyepiece" denotes the need to purchase appropriate eyepieces separately. Unlike astronomical telescopes, spotting scope eyepieces are generally not interchangeable between brands. Most spotting scope eyepieces cannot be removed, meaning that the magnification (or zoom range) cannot be changed.
For many years, fixed magnification eyepieces were optically superior, but zoom eyepieces have (mostly) closed the gap. The best of both worlds is a scope with interchangeable eyepieces that include both fixed magnification and zoom types. (The catadioptric Questar 90mm spotter would be one such example)
Waterproofing, fog proofing and armoring all pertain to the scope's ruggedness and durability under adverse conditions. Waterproofing prevents moisture penetrating the scope, while fog proofing will prevent condensation and fog on the inner surfaces when subjected to temperature changes. Armored bodies will withstand slightly more punishment on the trail. For extra protection, scope coats that allow use of the scope without removal are available. Note that none of these features are indicative of mechanical quality and even the cheapest spotting scopes are typically advertised as water, fog and impact proof.
Tripod mounts are best placed at the scope's balance point. Some models permit the scope to rotate on its axis via collared mounts, a nice feature if viewing from an unconventional position.
When choosing a scope, it is best to determine how it will be used and the features most important to that end. Then find a scope of the highest quality you can afford within those parameters.
For example, if the scope will be used in a dry environment and transported to the observation spot by vehicle, weight, waterproofing and armoring aren't so important. These features can be ignored in favor of a high quality, large aperture, high magnification scope. Bench rest shooters, for example, require premium optics and plenty of magnification, while weight and waterproofing are relatively less important.
If, however, the scope will be hauled on long treks in difficult terrain, as in hunting, a lighter, smaller aperture, armored scope is preferable. For any marine application, such as whale watching, water and fog proofing are indispensable, but weight is usually not an issue. Aquatic bird watching in marshes dictates a scope that is lightweight, water and fog proof. For any viewing purpose, excellent optical and mechanical quality remain highly desirable.
Comparing scopes is only worthwhile if conducted logically. Comparing differences in aperture size and magnification should be done with similar quality scopes. When comparing the optical performance of different contenders, the numerical specs, i.e. objective diameter and magnification, should be identical.
Price points should be similar. It would be foolish to expect a $300 scope to perform on par with a $3000 model, regardless of what the specifications promise. A quantitative comparison is much easier than a true qualitative field test. After determining your budget limitation, scopes of the same brand and series can be compared with relative ease based on their numerical specifications.
Of course, nothing compares to systematic testing in the field. Whenever possible, scopes with similar specs and comparable price levels from different brands should be evaluated side by side to see which one best fits your specific needs.
A rational assessment would consist of resolving an identical object at the same distance and testing under different lighting conditions. An open newspaper page taped to a wall can serve as a resolution test target. Pine needles and small twigs on hillside trees and bushes 100 yards or so distant are a good test of resolution, contrast and color fidelity. At night, point sources of light at the edge of the field make a good test for coma distortion.
Enjoying the great outdoors and interacting with nature is always laudable and (visually) getting up close is even better. Now is the time to gear up and get out. Spotting scopes have never been better or the choices more numerous.
Note: Reviews of Spotting scopes and binoculars can be found on the Scopes and Sport Optics index page.
Copyright 2014 by Joe Izrael and/or chuckhawks.com. All rights reserved.