Compact vs full-size binoculars: the weight saved and the light given up

The weight a compact saves

The chart holds 554 models. Of those, 94 have objectives under 30 mm and 188 have objectives from 40 to 45 mm. The compact median weight is 10.9 oz, and the full-size median is 24.9 oz.

Nikon says optics with a smaller lens aperture are lighter and more compact. Audubon says the same of objectives under 30 mm: lightweight and easy to carry. The figures in the chart agree.

If you want a pair that rides in a pocket, browse compact binoculars by weight and view for the models that carry the least.

The light a compact gives up

Exit pupil is the objective diameter divided by magnification. Nikon, ZEISS and the RSPB all give that formula, and Nikon says a larger exit pupil gives a brighter view in dim light.

The 10x25 size has an exit pupil of 2.5 mm and the 8x25 has 3.1 mm. The 8x42 has 5.3 mm and the 10x42 has 4.2 mm. You can work out any size with the exit pupil calculator.

Audubon says objectives under 30 mm are not bright enough to show detail in poor light. The RSPB describes 25 mm objectives as light, small models for daytime use, and advises testing the brightness.

Why daylight hides the gap

Nikon says the human pupil opens about 2 mm in daylight and 7 mm in the dark. Any exit pupil above about 2 mm looks bright enough by day, so a 2.5 mm compact holds up at noon.

At dusk the eye opens wider and the compact runs out of light first. ZEISS says the pupil is open only 2 to 3 mm by day, which is why the gap shows late in the day, not at lunchtime.

Field of view and the middle sizes

The catalog median field of view is 340 ft, with models running from 141 to 563 ft. The 8x25 median is 357 ft and the 10x25 median is 297 ft. The 8x42 median is 393 ft.

Between the two groups sit the 32 mm pairs. The 8x32 has a median weight of 18.5 oz, a 4 mm exit pupil and a 408 ft median field. The 10x32 weighs 19 oz with a 3.2 mm exit pupil. See 8x32 binoculars for those models.

Audubon says bird-worthy binoculars should have a field of at least 6.5 degrees, which is 341 feet at 1,000 yards. The 10x25 median of 297 ft falls short of that line.

Which size suits which job

Audubon recommends magnification of six, seven or eight times, and used 8x42 for its testing because most birders prefer 7x or 8x for brightness and a wide field. The 8x42 median weight is 24.2 oz. Compare it with the 8x25 compacts at 10.9 oz.

The RSPB suggests 8x for woodland, close range or poorer light, and 10x for open country or large expanses of water. For a bright day on open ground, a compact is enough. For woodland at first light, the full-size exit pupil of 5.3 mm helps.

Audubon notes that objectives larger than 42 mm are heavier. The 10x50 median is 29.3 oz, and the 7x50 median is 36.8 oz with a 7.1 mm exit pupil.

Age and the exit pupil

Practical Sailor, citing NIH data, reports a mean night pupil of 7.33 mm for ages 20 to 29 and 5.58 mm for ages 60 to 69. It adds that the average 50-year-old has a pupil aperture of 5 to 6 millimeters.

ZEISS says children's pupils can open to more than 8 mm, while they seldom exceed 4 mm in old age. An older eye cannot use a very large exit pupil, so the 7.1 mm of a 7x50 may go partly unused.

Weighing the two together

For a daytime walk, a trip where weight counts, or a spare pair in a coat pocket, the compact median of 10.9 oz wins on carry. The 8x32 against 8x42 comparison sets the middle size beside the full-size one.

For dusk, dense cover or long sessions where you want the widest light cone, the full-size median of 24.9 oz buys an exit pupil of 4.2 to 5.3 mm in the 42 mm sizes. The best binoculars compared list shows every size side by side.