Binocular magnification chart

What the first number tells you

In a size such as 8x42, the first number is magnification and the second is the objective lens diameter in millimeters, according to the National Audubon Society. So 8x42 magnifies eight times with 42 mm lenses.

ZEISS says the distance to the object shrinks by the magnification factor. Its example is a deer 400 m away that looks only 40 m away through 10x binoculars.

Two powers dominate the catalog. There are 225 models at 10x and 178 at 8x, against 53 at 12x and 34 at 7x. Powers such as 9x, with 4 models, and 14x, with 4, are rare.

Field of view shrinks as power rises

The median field of view is 391 ft at 7x and 393 ft at 8x. At 10x it is 325 ft, at 12x it is 273 ft, at 15x it is 231 ft and at 20x it is 171 ft. All figures are from the makers' documents.

Nikon defines real field of view as the angle of the visible field, seen without moving the binoculars. The National Audubon Society says bird-worthy binoculars should show at least 6.5 degrees, which is 341 feet at 1,000 yards.

Against that 341 ft mark, the 8x median of 393 ft clears it and the 10x median of 325 ft falls short. The widest 10x model reaches 472 ft, so the spread inside one power is large. The 7x range runs from 267 to 563 ft.

Use the field of view calculator to turn a printed figure into feet or degrees for any model.

Weight and steadiness at each power

Median weight is 21.8 oz at 8x and 22.9 oz at 10x. At 12x it is 28.8 oz, at 15x it is 45.1 oz and at 20x it is 55.9 oz. The 7x median of 32.2 oz is higher because several 7x models carry 50 mm lenses.

The National Audubon Society notes that binoculars magnifying eight times also magnify hand movement eight times. The British Astronomical Association says anything above 10x is difficult to hold and should be mounted.

Celestron puts the line at 15x: a tripod is not required from 8x to 12x but is necessary at 15x and higher. The 15x models have a median weight of 45.1 oz and a median field of 231 ft, so a mount is part of the plan.

Stabilized models change that. Fujifilm lists 12x40 and 16x40 Stabiscope models with 5 degrees of stabilization, and Nikon lists 10x25 and 12x25 stabilized models at 14.3 oz and 13.9 oz without batteries. See image stabilized binoculars for the models.

Matching power to what you watch

The RSPB shop suggests 7x or 8x for general and garden birdwatching and 10x for greater distances. It also says 8x suits woodland and poorer light, while 10x suits open country and large expanses of water. The 8x42 size and the 10x42 size cover those two cases.

For the night sky, the Royal Astronomical Society names 8x40 and 10x50 as the sizes usually recommended for starting out. The British Astronomical Association adds 7x50. Practical Sailor calls 7x50 the traditional marine size, with an exit pupil of 7.1 mm. See 10x50 binoculars and 7x50 binoculars.

ZEISS says free-hand binoculars typically run from 7 to 10x, and that lower power is the better long-run choice when in doubt. The best binoculars compared page sorts the full catalog, and the binocular size chart reads the objective side of the numbers.