How to Choose a Spotting Scope: Buying Criteria for Birders

Where a scope fits next to binoculars

The Missouri Department of Conservation puts the division of labor plainly: binoculars are usually the most useful tool for general bird observation, while spotting scopes are invaluable for long distance viewing, such as identifying shorebirds or monitoring an eagle nest. Audubon’s guide, written by field editor Kenn Kaufman, draws the same line. For backyard birding or pursuing warblers in the woods, binoculars may be all that is needed. A scope matters when scanning waterfowl far out on a lake, studying shorebirds on an open mudflat, or checking a hawk on a distant snag, where even the best binoculars do not resolve the bird.

That framing changes what to measure. A person who watches feeders and a garden hedge gains little from a scope. A person who visits lakes, tidal flats or raptor nests has a use for one. The same Audubon article adds that regular scope use shows details that would otherwise go unnoticed.

Objective lens size and weight

Binoculars are described first by magnification, such as 7X. Spotting scopes are not, according to Audubon, because their magnification is usually set by the eyepiece. The headline number is the diameter of the objective, the front lens, in millimeters. Audubon notes that many of the most popular birding scopes have objectives of 50 to 80 mm. Bigger lenses let in more light, but they add bulk and weight.

The Missouri guide sorts scopes into three sizes: compact and mid-sized scopes in the 12 to 45 power range, full-sized in the 20 to 60 range. It suggests compact or mid-sized scopes for beginning or average birders because they are lighter and easier to use, and adds that optical quality is sometimes not as good in the smaller ones. It also states that, on the same power, an 80 mm objective gives a brighter view than a 72 mm one.

Magnification and eyepieces

Higher power is not the goal. Audubon says that with a fixed eyepiece, an eyepiece of 20X to 30X will be most effective in most birding situations, since heat shimmer and vibration frequently obscure the view at higher magnification. The Missouri guide makes the same point from the other direction: most birders seldom use the 60X end of even their large scopes, because the range of vision narrows and heat waves, precipitation and a slight wind spoil the picture.

Zoom eyepieces get a favorable mention. Audubon calls them often the best for birding and lists 20-40X, 20-60X and 25-50X as popular ranges. The method it describes is to locate and center the bird at the lowest power, then zoom in as far as the shimmer allows. Some scopes have permanently attached eyepieces and others take detachable ones, so the choice is made at purchase.

Eye relief is the distance at which the eye can sit and still see the full image. The Missouri guide advises anyone who wears glasses to look for a high eye relief figure, usually above 15 or 16 millimeters, and notes that retractable eyecups are most often extended by people who do not wear glasses.

Glass quality and coatings

According to Audubon, all good scopes now use coated lenses and multi-coated lenses are the top standard. Low-quality glass produces chromatic aberration, a fringe of color around an object. Extra-low dispersion glass, abbreviated ED, avoids it. Audubon points out that makers use different terms, and that HD, XD or APO may stand for the same idea, so the term in a given brand’s specification needs checking. The Missouri guide likewise says ED or HD lenses reduce blurriness or colors at the edge of the field.

Audubon ranks these priorities clearly: when the choice is between better lenses and higher power, the sharpest and clearest image makes for the more satisfying experience.

Straight or angled body

Most good scope models come in both configurations, per Audubon. A straight body suits a person who usually birds alone and owns a sturdy, tall tripod, since the scope can be set at eye level for long scanning sessions. An angled body, looked into at about 45 degrees, suits a group: the height is set for the shortest person and taller people lean over a little farther. Audubon adds that an angled eyepiece works better when aiming up at birds overhead and is easier for field sketching, because the eye moves between scope and sketchbook with only a slight shift.

Focusing, field of view and close focus

Focusing is usually a ring or knob on the barrel. Some models have two-step focusing, with one control for a quick shift from close-up to infinity and another for fine adjustment. Audubon calls this a matter of preference and suggests checking how the mechanism feels in the hand.

Close focus gets its own paragraph in the same article. Birders are not always straining at distant specks, and a hummingbird at a feeder a few yards away needs the scope to focus near. Audubon says any birding scope should ideally focus down to 20 feet or closer.

Field of view is a number given on specification sheets, and the sources reviewed say little about it for scopes beyond the Missouri note on the narrowing range of vision at high power. Audubon’s binocular tips state that a wide field of view makes it easier to find birds and follow them in flight, which is a binocular statement and is not carried over to scopes here.

Tripod, mounts and support

A scope magnifies more than binoculars, so movement shows. The Missouri guide calls a solid base essential and recommends a strong, heavy tripod to reduce vibration. Audubon says a birding tripod need not be as rigid as one for photography but must be firm enough for a steady view in wind, and must extend far enough to place the scope at a comfortable height for long sessions. It also tells buyers to include the tripod in the budget from the start. The Missouri guide mentions a window mount as a useful alternative for viewing from a parked car, which works as a blind.

Weather and moisture protection

For scopes, the reviewed sources give no general rule on sealing. Audubon’s binocular tips say that binoculars should be waterproof and nitrogen purged so they do not fog in humid conditions. Whether a given scope offers that must be read from its own sheet. Celestron’s C90 table, for example, lists environmental protection as water resistant and nitrogen filled as no.

What a manufacturer sheet states about the Celestron C90

Searches for a Celestron C90 spotting scope lead to the maker’s product page, which calls the C90 MAK a catadioptric spotting scope: the optical design includes both a lens and a mirror, with light passing through a corrector lens to the primary mirror and back to a secondary spot before reaching the eyepiece. The page says this design does not need occasional collimation.

Its specification table lists a 90 mm objective, 39X with the included 32 mm Plössl eyepiece, a focal length of 1250 mm, an angular field of 1.3 degrees with that eyepiece, eye relief of 20 mm, close focus of 15 feet and a weight of 74 ounces. It lists a 45 degree erect image diagonal among the included items, and two 1/4-20 mounting holes plus a dovetail rail for a tripod. Those are the maker’s statements. This page does not judge how the model performs, rank it, or compare it with other scopes, and it carries no review scores.

Trying one before buying

The last item in Audubon’s article is practical: no amount of specification reading replaces handling a scope. Larger nature centers and wild bird stores sometimes keep several models, and birding festivals often have optics representatives with full product lines. Taking time to compare, in the field if possible, tests the focus feel, eye relief and tripod height together. Related reading: the Audubon section and the warranty note.