Key features to consider

When you're considering buying a spotting scope, pay particular attention to the following optical and design features:

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. Astronomy telescopes are much more powerful, but usually lack adequate light-gathering ability for bird watching and are not weather-proof for field use.

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 then a quick shift to higher power for looking at details. Like binoculars though, scopes suffer from less light, narrower field of view, and more vibration as magnification increases. The scope's optical quality becomes apparent when you zoom beyond 45x: in cheaper scopes the image generally becomes too dark to see much detail as you move toward 60x, making viewing in less-than-perfect lighting conditions more difficult. This was especially true of the zoom lenses of older scopes. However, in today's better quality zooms the loss of image quality at high powers may be negligible. 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 indistinguishable from that 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 and at high power. You should base your decision whether to go the high quality glass, high price route on the kind of birding you plan to do.

Light-gathering capacity

Like binoculars, the light-gathering capacity of a spotting scope is reflected by the size of the objective lens (the one farthest from your eye). Depending on the model, this value may be from around 50mm to 100mm or more, with larger objective lenses providing brighter images in general. However, large objective lenses also make scopes heavier.