Optics & Night vision

Optics and night vision, read from the two numbers on the barrel

The optics & night vision range gathers daytime binoculars, digital night vision units, monoculars and polarised eyewear. Almost every instrument in it is described by two numbers, and reading them removes most of the guesswork: magnification first, then the diameter of the front lens in millimetres. Everything else on the specification sheet follows from those two figures and from what the coatings do with the light they let in.

What do 8x42 and 10x42 actually mean?

The first number is how many times closer the subject appears. The second is the objective diameter, which governs how much light reaches the eye. Divide the second by the first and you get the exit pupil, the width of the beam leaving the eyepiece. A wider exit pupil is easier to hold on target and stays usable at dusk. That is why 8x42 binoculars feel calm and bright in hand and forgiving of a shaky grip. Step up to 10x42 binoculars and the image is larger but dimmer and less steady, which is the trade you accept for reach.

How does night vision differ from a daytime optic?

A daytime binocular only gathers existing light, so once the light has gone there is nothing to gather. Digital night vision works differently: a sensor reads the scene, an infrared illuminator throws light the eye cannot see, and the result is shown on a small internal screen. That is why a unit needs batteries and why the image is rendered rather than optical. It also means the illuminator has a working range beyond which the picture goes dark, so units in the night vision binoculars group are used at distances well short of what their daytime magnification suggests.

Which specification matters for the way you will use it?

• Field of view: the width of scene visible at a set distance. High magnification narrows it, which makes finding a moving subject harder.
• Eye relief: the distance the eye can sit behind the lens. Spectacle wearers need a generous figure and twist-down eyecups.
• Close focus: the shortest distance the instrument will focus at. It matters for birds and insects, not for landscapes.
• Sealing: nitrogen or argon filling displaces damp air inside the barrels, which stops internal fogging when the temperature changes.

Binoculars or a monocular?

Two eyes give depth and comfort over long periods, which is why binoculars remain the default for watching rather than for checking. A single barrel halves the weight and the bulk, fits a coat pocket and is quicker to raise, at the cost of tiring one eye during extended use. A monocular is the sensible choice when the instrument is carried all day and used in short bursts.

How do you keep optics working in British weather?

Fogging is usually external and clears on its own. Fogging inside the barrels means the seal has gone, and wiping will not help. Let a cold instrument warm up in its case before opening it, rather than exposing chilled glass to a warm room. Blow grit off before touching a lens, since dragging a cloth across sand leaves permanent marks on a coating. Store with the caps on, somewhere dry, and not in a car boot through winter.