Binoculars

Read the two numbers before anything else

Binoculars are described by a pair of figures such as 8x42 or 10x42, and those two numbers tell you almost everything: the first is the magnification, the second the diameter of the front lens in millimetres. The range covers pocket models, the classic 8x42 and 10x42 formats, high-power designs, nitrogen-filled waterproof bodies and digital night vision units. Once the numbers are understood, the choice narrows to how you plan to use them. The pocketable end sits in compact binoculars.

Does more magnification give a better view?

Not on its own. Magnification enlarges the image and every tremble of your hands with it, so a high-power pair held freehand often shows less detail than a lower-power one held steady on the same subject. It also narrows the field of view and darkens the image unless the front lens grows to match. This is why 8x is the format many settle on, and why anything stronger asks for a tripod or something to lean on.

Why does the front lens size matter?

The front lens gathers the light. Divide its diameter by the magnification and you have the width of the beam leaving the eyepiece, so 8x42 gives a wider beam than 10x42 from the same body. A wider beam means a brighter image at dawn, at dusk and under trees. At midday the difference is hard to see, which is why small lenses are usable in daylight and disappointing at first light.

Which format for which use?

• Compact: small lenses and a folding body that fits a jacket pocket, suited to daylight and to being carried every day.
• 8x42: a wide field and a bright image, the format that stays steady in the hand for long watching.
• 10x42: more reach for detail at distance, at the cost of a narrower view and less tolerance of shaky hands.
• High power: strong magnification for fixed observation, realistically used with a support.
• Digital night vision: a sensor and a screen instead of glass, showing an image in darkness that no optical pair can.

The wide-field format sits in 8x42 binoculars. The longer-reach one is grouped as 10x42 binoculars.

What does nitrogen filling do?

Air holds moisture, which condenses on the inner surfaces when the temperature changes and fogs the view where no cloth can reach. Filling the body with dry nitrogen removes it, so the optics stay clear when a warm pair meets cold air. It usually comes with sealed housings that keep rain and dust out, though it is not the same as a rating for immersion; the product page states what a given model claims.

How do you set them up for your eyes?

Two adjustments, done once. Fold the barrels together or apart until the two circles merge into one, setting the spacing to your eyes. Then close the right eye and focus the centre wheel until the left side is sharp; close the left eye and turn the dioptre ring until the right side matches. After that only the centre wheel is used, and spectacle wearers should roll the eyecups down to restore the full field of view. Sensor-based units in night vision binoculars focus differently, using a screen rather than the eye.