8x42 or 10x42 binoculars for birding? Why 8x42 fits most NYC walks

The short answer

Choose 8x42 unless you spend most of your time watching distant birds on open water. 8x42 is on the list of birding sizes in Audubon's buying guide, "How To Choose Your Binoculars." 10x42 is not on that list. The same guide recommends six-, seven- or eight-power binoculars for birding.

The reasons are practical. At 8x you get a brighter image, a wider view and less shake from your hands. In a park full of moving warblers, those three things matter more than seeing a bird a little larger.

10x42 has one real advantage: more detail on a bird that sits still far away. If that describes your usual birding, it can be the right tool. For everything else, start with the chooser on this page at the 8x setting; the full binocular chooser has more presets.

What the numbers mean

The two numbers are magnification and the objective lens. An 8x42 makes a bird look eight times closer and has front lenses 42 mm across. A 10x42 has the same objective size but makes the bird look ten times closer.

The exit pupil is the width of the beam of light that leaves the eyepiece, in millimeters. It is the objective divided by the magnification, and relative brightness is the exit pupil squared. Here is the arithmetic once:

• 8x42: 42 ÷ 8 = 5.25 mm exit pupil; 5.25² ≈ 27.6 relative brightness.
• 10x42: 42 ÷ 10 = 4.2 mm exit pupil; 4.2² ≈ 17.6 relative brightness.

So the 8x42 has about one and a half times the relative brightness of the 10x42. You notice it most under a dense canopy, on a gray morning or at dusk.

Twilight factor is the square root of magnification times objective. It gives about 18.3 for an 8x42 and about 20.5 for a 10x42. It rewards magnification, so read it together with exit pupil, not instead of it.

Field of view is how wide a scene you see. Spec sheets give it either as feet at 1,000 yards or as an angle in degrees. To convert, Audubon says to multiply the angle by 52.5. Its minimum for birding is 6.5°, which works out to 341 feet at 1,000 yards. A 7° model, for example, gives 7 × 52.5 = 367.5 feet.

Eye relief is how far your eye can sit from the eyepiece and still see the whole view. Audubon puts binoculars designed for birders at 15 to 20 mm. This number matters most if you wear glasses.

Which fits NYC birding

In wooded parks, 8x42 wins. In Central Park and Prospect Park, most birds are close, quick and half hidden in leaves. Black-throated Blue Warbler is common in NYC reports in May and September, and American Redstart in May and from August to September. Finding a bird fast takes a wide, steady view, and Audubon notes that ten-power binoculars cut the field of view.

On open water, the case for 10x42 is stronger. At Jamaica Bay, birds often sit far out and stay put. Brant and Bufflehead are common in NYC reports from November to March, and Semipalmated Sandpiper in August. Extra magnification helps with distant ducks, but Audubon warns that higher power also makes heat shimmer worse across a marsh on a warm day.

For a yard or a balcony, magnification hardly matters, since your feeder birds are a few yards away. Weight and comfort matter more. A lighter 8x32 or a compact may suit you better, as our page on compact binoculars explains.

Editorial tip: if you bird both woods and water, buy the 8x42 first. When distance becomes the problem, a spotting scope on a tripod adds far more reach than two extra powers in your hands.

What to check before you buy

Hold the binoculars before you commit, whatever the magnification. Audubon's guide covers four checks you can make in the store.

Focus comes first. Look for one center focus knob between the barrels that runs from close to far in about one to one and a half turns. Separate focus on each barrel is too slow for birds.

If you wear glasses, wear them in the store. Set the eyecups fully retracted and make sure you see the whole circle of the view, edge to edge. Then set the right-eye diopter: focus on something 50 to 100 feet away with the left eye, then match the right eye with the diopter ring.

Feel the weight and the grip. Both configurations use 42 mm objectives, and Audubon calls larger objectives too heavy for most people to carry all day. Check that you can reach the focus knob without shifting your hands.

Last, the budget line. Audubon says binoculars above $200 should be waterproof and nitrogen-purged, so they do not fog in humid weather. Below $200, focus, eye relief and field of view matter more than any extra feature, as our guide to budget binoculars explains.

Mistakes to avoid

Buying the highest magnification you can find is the most common mistake. Audubon warns that too much magnification makes binoculars useless, because every hand tremor is magnified too. It adds that ten-power or higher binoculars are tiring to use, narrower in view and less bright than seven- or eight-power models.

Do not buy zoom binoculars or image-stabilized binoculars for birding. Audubon calls them heavy, notes that they lose a lot of brightness, and says they have a much-reduced field of view.

Do not judge a pair by magnification alone. Compare exit pupil, field of view and eye relief side by side; the optics section and our page for beginners walk through the rest.