A spotting scope is a small telescope that has been modified for use by day. A spotting scope differs from an astronomical telescope in several important ways. First, a spotting scope always produces an upright image whereas a telescope used in astronomy may produce a reversed image or even an upside down image (not a problem for astronomy). Second, a spotting scope is much smaller in size than an astronomical telescope, mainly for the sake of portability. Third, a spotting scope is a lower magnification instrument than a telescope, since the atmosphere by day does not allow the high magnifications used in astronomy. Fourth, a spotting scope is mounted on an ordinary photo tripod, but a telescope for astronomy requires a very specialized mount, often unsuitable for daytime use. Lastly, many, if not most, spotting scopes are waterproof and fogproof - a rare feature in an astronomical telescope.
Spotting scopes are used anytime you need more magnification than a binocular provides. Spotting scopes are widely used for birding, surveillance, hunting, and viewing landscape, wildlife, ships and other distant objects. Spotting scopes are also used for scoring targets on rifle, pistol and archery ranges and they can also be used to some astronomy. Last, but not least, spotting scopes are also used to take long distance pictures with a variety of cameras.
Most spotting scopes are labeled with three numbers. The first two numbers represent the magnification range and the last number is always the size of the front lens. For a 15-45x60 model, then, you have a spotting scope with a zoom magnification range of 15-45x with a front lens of 60mm diameter.
Spotting scopes begin in magnification where conventional binoculars stop. The higher magnifications offered by a spotting scope allow you to view birds, wildlife, scenery and other objects that are well beyond the range of a conventional binocular. However, there are limits to magnification. Two things determine how much magnification you can use in a spotting scope.
The first is the atmosphere. You must always factor in the seeing conditions of the atmosphere when using a spotting scope. Heat waves, dust, humidity, glare, wind and air currents during the day all reduce image quality and the greater the magnification, the more drastic the reduction in image quality. Simply put, there will be days when anything over 30x will appear as mush, but there will also be those clear, calm days when you can use 60x to good advantage. In general, high altitudes and dry climates favor high magnification whereas wet, humid, low-altitude climates discourage high magnification. Very few locations, however, allow you to use more than 60-80x during the day, so most spotting scopes stop at 60x. Many beginners make the mistake of buying an astronomical telescope that can magnify hundreds of times for day use, only to discover that the atmosphere still limits them to around 60x and quite often, much less.
The atmosphere, for the same reason, also limits how much detail you can see at great distances. Seeing tiny detail, such as a person's facial features, at a mile or more through an ocean of turbulent, ever-moving air is simply not possible, no matter how much magnification you use or no matter how large or expensive the instrument. Spotting scopes are great, but they can't work miracles. On the other hand, it is quite possible to view large objects such as ships at this distance, though the image may be blurry or wavy.
The second major limit on magnification is the optical system of the spotting scope, itself. Regardless of the model, there will always be some drop off in image quality as magnification goes up. This is determined somewhat on the design and size of the scope, but primarily on the quality of the optical system. Here you get what you pay for in a very visible way. Inexpensive spotting scopes, regardless of size or type, lose image quality quickly as magnification goes up, but premium grade scopes lose very little in image quality, even at the highest magnification. Be prepared to pay, however, if you want a spotting scope that is as sharp at 60x as it is at 20x. Only a handful of very expensive spotting scopes are capable of this.
Most observing with a spotting scope, though, is done at lower magnifications, usually around 30x-40x. This is more than enough for most applications and all but the cheapest model will produce reasonable images in this magnification range.
The larger the objective lens, the more detail you can see and the better image quality your scope will deliver, especially at higher magnifications, assuming you are comparing two models of similar quality. However, a large lens of mediocre quality, no matter how large, will never equal the performance of a smaller, quality lens. When in doubt, go for quality, not size.
You can also improve the performance of any spotting scope, especially at higher magnifications, with special quality glass (ED glass, APO glass, HD glass, Fluorite glass) in the objective. This option will cost you more, but it can deliver image quality on a par with a larger objectives made of standard glass. If portability is an issue, a smaller model with ED glassor other special glass may be all the spotting scope you will ever need.
If you need the absolute best in performance, however, there is still no substitute for a large, quality objective made with high performance glass.
Although not an absolute necessity, a spotting scope with a close focus of less than 20 ft can be useful, especially if you plan to use a spotting scope with a camera.
This is the distance your eye can be from the eyepiece and still see the entire field of view. This is an important feature if you wear eyeglasses when you observe or if you are not in a position to get close to the eyepiece. As a general rule, you will need at least 14mm of eye relief to see the entire field of view with eyeglasses and people with thick glass lenses in their eyeglasses will probably need more.
Lens coating improve light transmission which is important in a high magnification instrument such as a spotting scope. In order of increasing quality and performance, you will find "fully coated" , then "multi-coated", then "fully multi-coated". All premium grade models are fully multi-coated, often with special chemical formulas that vary from manufacturer to manufacturer.
"Keep your expectations in line with price and buy according to your needs," is excellent advice for any optical instrument, but it applies especially to spotting scopes. There is a huge gap in performance and price between low quality scopes and premium grade scopes. For many applications, you can get by with a low or medium priced model, but if you need the performance of a high grade model, there's no getting by cheap. You must pay the price.