Magnification Power
Spotting scopes are medium-range telescopes, usually with a magnification power between 15x and 60x. To change magnification power, they have either interchangeable fixed-length eyepieces or a single zoom eyepiece.
When you’re scanning an area with a spotting scope it’s best to start with a low power eyepiece or the lowest setting on a zoom eyepiece (for instance in the 20x to 30x range). Once you’ve located the birds you want to examine closely you can switch to higher power.
Zoom Lenses
Zoom lenses change magnification power from 20x to as high as 60x with a single, simple adjustment. They offer a definite advantage for bird watching, allowing convenient scanning at low power and a quick shift to higher power for looking at details. But as with camera lenses, zoom lenses don’t gather light as well as fixed lenses. Also, as magnification increases, any scope (or binocular) will suffer from less light, narrower field of view, and more vibration. High powers also magnify the effects of haze and shimmering heat distortion seen over water and other flat expanses.
Twenty years ago, a good zoom lens was hard to find, and the costs (both optically and dollar-wise) were large. Nowadays, many mid-priced scopes have excellent zoom lenses. At high power, top quality zooms give image sharpness and clarity almost as good as at low magnification, so buy the highest-quality scope you can afford.
Glass Quality
Top spotting scope lenses are made with fluorite-coated, HD (high density), or ED (extra-low dispersion) glass. The difference in brightness and image clarity between these high-quality scopes and those made by the same manufacturers using standard glass is particularly noticeable in low-light viewing conditions (such as in late evening) and at high power. You should base your decision whether to go with high quality, high priced glass on the kind of birding you plan to do.
Light-Gathering Capacity
Like binoculars, the light-gathering capacity of a spotting scope is indicated by the size of the objective lens (the one farthest from your eye). Depending on the model, this value is typically between about 50 mm and 100 mm. Larger objective lenses providing brighter images in general, but they also make scopes heavier and harder to pack in luggage.
Eyepiece Placement
Another consideration when selecting a spotting scope is eyepiece placement. Some scope models have eyepieces configured for straight-through viewing, making it easy to quickly locate and follow a subject. This seems like a natural design, but many bird watchers prefer a different approach, the 45-degree angled eyepiece. This style makes viewing above the horizon easier, works with shorter tripods (which are inherently more stable), and makes birding much more convenient when you’re in a group of people of different heights.
Eye Relief
Eyeglass wearers should pay attention to the amount of eye relief offered by the scope. With longer eye relief, the optics direct the focal point farther back behind the eyepiece so the eyeglass wearer can see a complete field of view. Eye relief is given in millimeters in the model’s technical specifications. Generally, 12–15 mm of eye relief is adequate for most eyeglass wearers. As with binoculars, some scope designs have folding or moveable rubber eyecups to accommodate non-eyeglass wearers.
Focusing Mechanism
In spotting scopes, focusing is normally done in one of two ways. With a focusing collar, the whole barrel of the scope is knurled or rubberized and you just twist the whole barrel to make the image sharper. The other design uses a smaller focus knob typically mounted on the top of the scope near the eyepiece. These are slower to use but permit more precise focusing. Your hand size and dexterity may be an issue here, so try each style to find your preference.