Every pair of binoculars carries two numbers, such as 8x42 or 10x42. The first is the magnification and the second is the diameter of each front (objective) lens in millimeters (Cornell Lab of Ornithology). This page works through what those numbers imply, what the other specifications on a box mean, and where Audubon, the Cornell Lab and bird-club guides give different advice. It names no models and gives no prices. For first steps in birding and how to set a pair up for your eyes, see how to start birding.
Quick read
8x42: wider view, steadier image, 5.25 mm exit pupil. The usual birding advice (Cornell, Audubon).
10x42: the bird looks 25% larger, but hand shake is magnified and the view is usually narrower; the exit pupil drops to 4.2 mm.
Then check: field of view (at least 6.5°, per Audubon), close focus (10 ft or less, per Cornell), eye relief if you wear glasses, and waterproofing.
8x42 vs 10x42 side by side
Both have the same 42 mm objective lenses, so they gather the same light. The difference is how much the image is enlarged, and what that costs.
| 8x42 | 10x42 | |
|---|---|---|
| How much larger the bird looks | 8 times | 10 times (25% more than 8x) |
| Exit pupil (42 ÷ magnification) | 5.25 mm | 4.2 mm |
| Relative brightness (exit pupil²) | 27.6 | 17.6 |
| Twilight factor (√ of 8×42 or 10×42) | 18.3 | 20.5 |
| Field of view | Wider: "Field of view decreases with higher magnification" (Cornell) | Narrower; Audubon's Wayne Mones says 10x and higher "slash the field of view" |
| Hand shake | Less visible | Magnified: "The more you magnify the image, the jumpier it will get" (Baltimore Bird Club) |
| Who it suits, per sources | Cornell: many birders prefer 8x; Baltimore Bird Club: even expert birders "usually won't choose magnification higher than 8x" | Columbus Audubon: birders with steady hands may prefer 10x for distant subjects |
Mones adds that higher power makes heat shimmer worse, dims the image compared with 7x or 8x, and is more tiring to use. Seattle Audubon's optics guide calls 10x the "highest power that can be comfortably handheld," and Wikipedia gives 7x to 10x as the usual hand-held range. Image-stabilized binoculars can be held steady at up to 20x (Wikipedia), but Mones advises against them for birding: they are heavy, lose brightness and have a narrower field of view.
The numbers behind brightness
Exit pupil is the beam of light leaving each eyepiece: objective diameter divided by magnification (Wikipedia; Columbus Audubon). It only helps up to the size of your own pupil. Wikipedia gives about 3 mm for a pupil in daylight and about 7 mm at night, and says pupils of people over 50 rarely open beyond about 5 mm. In daytime, light beyond your pupil's size is wasted, so on a bright day both an 8x42 and a 10x42 more than fill the eye. The larger exit pupil counts in poor light; Wikipedia notes that birders choose 40 to 45 mm objectives for low light and for seeing into foliage. Columbus Audubon puts the useful range for birding binoculars at roughly 3.5 to 6 mm.
Relative brightness is the exit pupil squared. Wikipedia calls it a mainly historic measure. Twilight factor is the square root of magnification times objective diameter. It rises with magnification, so it favors the 10x42 (20.5 against 18.3), but Wikipedia warns it does not describe low-light performance on its own, without the exit pupil.
| Size | Exit pupil (mm) | Relative brightness | Twilight factor |
|---|---|---|---|
| 7x42 | 6.0 | 36.0 | 17.1 |
| 8x42 | 5.25 | 27.6 | 18.3 |
| 10x42 | 4.2 | 17.6 | 20.5 |
| 8x32 | 4.0 | 16.0 | 16.0 |
| 10x32 | 3.2 | 10.2 | 17.9 |
All five sizes are named in the Audubon, Cornell or Baltimore Bird Club pages cited below; the figures are worked out with the formulas above.
- 8x42: 42 ÷ 8 = 5.25 mm exit pupil. The dashed yellow ring in each blue disc is a 3 mm daylight pupil.
- 10x42: 42 ÷ 10 = 4.2 mm.
- 8x32: 32 ÷ 8 = 4.0 mm.
- Daylight pupil, about 3 mm (Wikipedia; the Baltimore Bird Club guide says 2 to 3 mm). All three binoculars more than fill it.
- After 50, pupils rarely open past about 5 mm (Wikipedia); Columbus Audubon puts the useful limit for older eyes at about 4 mm.
- At night, about 7 mm (Wikipedia).
Spec calculator
Enter the two numbers from any binocular, and the field of view in degrees if the box gives it.
The calculator uses these formulas:
relative brightness = exit pupil × exit pupil
twilight factor = √(magnification × objective)
field of view (ft at 1,000 yd) = degrees × 52.5
The first three come from Wikipedia's binoculars article; the 52.5 conversion is from Mones' Audubon guide. It matches Cornell's table: 6.5° is 341 feet and 8° is 420 feet.
The other numbers on the box
Field of view. The width you see, given in feet at 1,000 yards or in degrees (Cornell). Cornell puts typical birding binoculars at 5.5° to 8.5°, or 290 to 470 feet, and says each extra half degree gives about 8% more width. A wide view makes birds easier to find, especially for beginners. The stated figure is a best case: eyes held farther back, as with glasses, see less (Cornell).
Close focus. The nearest distance that looks sharp. Cornell says many birding binoculars focus to 10 feet or less and some to about 5 feet, and that close focus is not always tied to price. Columbus Audubon calls 10 to 12 feet or less "highly desirable" and suggests avoiding more than 15 feet.
Eye relief. How far your eye can sit from the eyepiece and still see the whole field. Mones gives 15 to 20 mm for birding models. Wikipedia says eyeglass wearers generally need more than 16 mm, or more than 17 mm with thick frames. Columbus Audubon notes that manufacturers measure it differently, so try a pair with your glasses on.
Interpupillary distance. The hinge sets the gap between the eyepieces. Most adults fall between 50 and 75 mm (Wikipedia); adjust until you see a single circle (Columbus Audubon). Some children and narrow-set eyes may not fit large roof-prism models (Wikipedia).
Weight. Columbus Audubon gives 22 to 30 ounces as typical. The Baltimore Bird Club guide says heavier pairs hold steadier but tire arms on a long walk; Mones says objectives above 42 mm are too heavy for most people to carry all day.
Prisms, coatings and waterproofing
Porro prisms (patented by Ignazio Porro in 1854) offset the objectives from the eyepieces (Wikipedia). Roof prisms have a straight-through shape (Baltimore Bird Club), but their prism angles must be correct within about 2 arcseconds, which makes them costlier to produce (Wikipedia). Roof prisms split the light beam and need phase-correction coatings, developed in 1988; Porro prisms do not (Wikipedia).
Coatings reduce reflections. Wikipedia defines four levels: coated (some surfaces, single layer), fully coated (all air-to-glass surfaces), multi-coated (some surfaces, several layers) and fully multi-coated (all surfaces, several layers). A single magnesium fluoride layer cuts reflection from about 4% to 1.5% per surface; modern multi-layer coatings reflect 0.25% or less. Coatings cannot be replaced, and Mones warns that paper towels, wood-fiber tissues and household glass cleaners can damage them.
Waterproofing. Sealed binoculars with O-rings, filled with nitrogen or argon, resist internal fogging, tarnishing and lens fungus (Wikipedia). Mones advises that all but the least expensive binoculars be waterproof and nitrogen purged to prevent fogging in humid weather.
Where sources disagree
| Question | One view | Another view |
|---|---|---|
| Best magnification | Mones (Audubon): 6x to 8x, prefers 7x | Columbus Audubon: 7x or 8x all-round, 10x for steady hands at a distance; Seattle Audubon: most birders use 7x to 10x |
| Is 10x always narrower? | Cornell: field of view decreases with magnification | Baltimore Bird Club: "not EXACTLY"; some 8x binoculars show no more than some 10x |
| Useful exit pupil for older eyes | Wikipedia: over 50, rarely beyond about 5 mm | Columbus Audubon: about 4 mm |
| Which prism type is lighter? | Columbus Audubon and Baltimore Bird Club: roof prisms are lighter and more compact | Seattle Audubon: Porro models are slightly lighter |
| Value and sealing | Columbus Audubon and Baltimore Bird Club: Porro gives more optical quality for the money; roof is easier to waterproof | Wikipedia: Porro alignment may drift and need re-collimation; Baltimore: roof is less prone to being knocked out of alignment |
| Typical field of view | Cornell: 290 to 470 ft for birding models | Columbus Audubon: 350 to 450 ft for 7x and 8x |
Before you choose
- Decide 8x or 10x by trying both: can you hold 10x steady?
- Exit pupil in the 3.5 to 6 mm range Columbus Audubon gives for birding
- Field of view at least 6.5°, or 341 ft at 1,000 yards
- Close focus of 10 ft or less
- Eye relief that shows the full circle with your glasses on
- Hinge closes or opens far enough to give one round image
- Waterproof and nitrogen or argon purged; coated optics (fully multi-coated is the highest of the four levels)
- Weight you can carry for a full morning
For a Bird-a-Day streak, Cornell's point about field of view applies: a wide view helps with birds in flight or moving through dense foliage.
Sources
- Field of View Is Important in Birdwatching Binoculars (2022, updated 2023) — All About Birds, Cornell Lab of Ornithology
- Why Do Birdwatchers Need Close Focus in Binoculars? (2023) — All About Birds, Cornell Lab of Ornithology
- How To Choose Your Binoculars (Wayne Mones, 2016; first published 2009) — National Audubon Society and The 2013 Audubon Guide to Binoculars (Wayne Mones)
- A Beginner's Guide to Binoculars for Birding (undated) — Columbus Audubon
- Advice on Choosing Birding Optics (PDF, Pete Webb, 2013) — Baltimore Bird Club
- Choosing and Using Optics (PDF, undated) — The Nature Shop at Seattle Audubon (Birds Connect Seattle)
- Binoculars — Wikipedia (exit pupil, twilight factor, relative brightness, pupil sizes, prisms, coatings, sealing; secondary source)