Nikon ACULON A211 10-22×50 Zoom Binoculars: Specs and Stargazing Tips

The original post on this site was a short product description of the Nikon ACULON A211 10-22×50 zoom binocular (product 8252). This rewrite uses Nikon’s product page and the ACULON A211 instruction manual to explain the specifications and how to get the best from a zoom binocular.

Verified specifications

From Nikon USA and the ACULON A211 manual (figures marked at 10x are given by Nikon at the minimum zoom setting):

• Magnification: 10-22x zoom; objective diameter: 50 mm
• Porro prism, central focusing
• Real angular field of view: 3.8° at 10x; apparent field 36.7°
• Field of view: 199 ft at 1,000 yards at 10x
• Exit pupil: 5.0 mm at 10x, down to 2.3 mm at 22x
• Eye relief: 8.6 mm at 10x
• Close focusing distance: about 49.2 ft (15 m)
• Interpupillary distance: 56-72 mm
• Size 7.8 x 7.8 in; weight 33.9 oz (960 g)
• Supplied accessories include the TRA-2 tripod adapter
• Not waterproof: Nikon says to avoid getting it wet from rain or splashes

How the zoom works

Nikon’s manual says magnification is changed by moving the zoom lever. It advises focusing correctly at a higher magnification first, which minimises blurring when you change magnification. Nikon’s choosing guide warns that as zoom magnification goes up, the field of view narrows and hand shake becomes more noticeable; the exit pupil also shrinks, from 5.0 mm at 10x to 2.3 mm at 22x, so the image gets dimmer.

What the numbers mean

Every binocular carries two numbers. Astronomy magazine’s binocular guide explains that the first is the magnification and the second is the diameter of each front lens in millimetres, so a 10-22×50 zooms from 10x to 22x and has 50 mm lenses.

• Exit pupil: aperture divided by magnification. At 10x, 50 ÷ 10 gives 5.0 mm; at 22x it drops to about 2.3 mm.
• Why exit pupil matters at night: the same guide says most people under 30 have dark-adapted pupils of about 7 mm, around 6 mm in their 30s, and probably no more than 5 mm from age 40. A bigger exit pupil than your eye can use simply wastes light.
• Field of view: to convert feet at 1,000 yards into degrees, divide by 52.5. 199 ft at 10x is about 3.8°, which is narrow for a 10x binocular and narrower still when zoomed in. Astronomy magazine says most binoculars cover about 6° to 7°, high-power models 3° to 5°, and wide-angle models 8° to 10°.
• Eye relief: Nikon’s guide says eye relief of 15 mm or longer lets most eyeglass wearers see the whole field without removing their glasses. At 8.6 mm, this binocular is not ideal for glasses; Nikon’s manual says eyeglass wearers should fold back the eyecups.

Is it good for stargazing?

At 10x, the 50 mm lenses give a bright 5 mm exit pupil, good for the Moon and star clusters. Higher zoom settings go well beyond the 10x that Astronomy magazine suggests for hand-held use, and it notes that binoculars above 10x with large apertures need a tripod. Nikon supplies the TRA-2 tripod adapter with this model, so use it: at 22x, a tripod turns a shaky view into a useful one for the Moon.

Setting up your binoculars

Bushnell’s Trophy owner’s guide describes the standard set-up that works for most centre-focus binoculars:

• Eyecups: twist them up if you do not wear glasses, so your eyes sit at the right eye relief distance and you see the full field of view.
• Interpupillary distance: point the binoculars at a blank wall or sky and move the two halves closer or farther apart until you see a single circle with no shaded areas.
• Focus the left side: cover the right objective lens, then turn the centre focus knob until a detailed distant object looks sharp through the left side.
• Set the diopter: cover the left lens instead and turn the diopter ring under the right eyepiece, not the centre knob, until the right side is sharp.
• Remember your setting: note the diopter mark so you can reset it if someone else uses your binoculars.

What you can see in the night sky

According to Astronomy magazine, binoculars show the Moon in crisp detail, including shadows moving across lunar features as it waxes and wanes and earthshine on a crescent Moon. They also let you follow the phases of Venus and the motions of Jupiter’s big moons. Beyond the solar system, large star clusters such as the Pleiades in Taurus and the Beehive in Cancer fit nicely in a binocular view, and the Andromeda Galaxy (M31) stands out from a dark site. The magazine says more than 400 deep-sky objects lie within reach of good binoculars from a dark-sky site.

Care and cleaning

The Trophy guide’s cleaning advice applies to any binocular: blow or brush away dust first, then wipe in a circular motion with a microfibre cloth. For a thorough clean, use photographic lens tissue and lens-cleaning fluid, always applied to the cloth and never directly on the lens. Never use household window cleaner or paper towels, which can damage coatings or scratch the glass. Store the binoculars in a cool, dry place.

Safety

Never look at the Sun through binoculars or any optical instrument. Bushnell’s guide warns this may permanently damage your eyes.

Quick summary

• 10-22x zoom with 50 mm lenses; Porro prism.
• 3.8° (199 ft) field at 10x; 5.0-2.3 mm exit pupil.
• 8.6 mm eye relief at 10x; not waterproof.
• TRA-2 tripod adapter supplied; use it at high zoom.

Jargon buster

• Objective lens: the big front lens that collects light.
• Exit pupil: the small bright circle of light you see in each eyepiece when you hold the binoculars away from your eyes.
• Diopter: an adjustment for differences between your two eyes.
• Roof vs Porro prism: roof models have straight barrels; Porro models have the classic zigzag shape.