Binoculars comparison:the right pair for how you watch.

Two numbers sum up a pair, for example 8x42. The 8 is the magnification: the bird looks 8 times closer. The 42 is the diameter of the front lenses, in mm. The bigger it is, the more light the pair gathers at dawn or under the trees.

People often think more magnification is better. Wrong. Past 10x, the image shakes the moment your elbows aren't braced. The 42 mm is the versatile standard: drop to 32 mm to travel light, climb to 50 or 56 mm for the fading light of dusk.

It's the little circle of light that comes out of the eyepiece. You work it out simply: objective lens diameter divided by magnification. An 8x42 gives 5.25 mm.

At dawn, your own pupil opens up to 5 to 7 mm. If the exit pupil is smaller, the image looks dark. For low light, aim for at least 4 mm, ideally 5. In broad daylight it makes no difference: your pupil is closed down.

ED or HD glass reduces the colour fringing you get on high-contrast objects. A genuine plus on premium models, not magic on entry-level ones.

Coatings boost light transmission and contrast. The roof prism gives a slim, compact barrel (the modern standard), the Porro gives lovely depth for less money. Neither is better in absolute terms.

The width of what you see, in metres at 1000 m. The wider it is, the easier it is to follow a bird in flight or sweep across a landscape. A good 8x42 often sits around 130 to 140 m, and premium models climb beyond 150 m.

A wide field is a precious luxury in birdwatching: you find your subject faster and keep it in view when it moves. To compare two pairs, always look at the field at the same magnification. An 8x will almost always be wider than a 10x.

A pair that's too heavy stays at home. A 42 mm often weighs 650 to 850 g, a 32 mm can drop below 500. Past an hour of watching, the difference shows in your neck and your arms.

Ergonomics also means the focus wheel (smooth or not) and the eye relief: if you wear glasses, aim for at least 15 mm, otherwise you lose the edges. My advice: between two close models, take the lighter one, you'll use it more often.

Outdoors there's rain, mist and dust. A good pair is waterproof and nitrogen-purged, which stops internal fogging when you move from cold to warm. Most serious models are these days: it's an insurance, not a marketing line.

The rubber armour protects against knocks and improves grip, especially with gloves. For the mountains or winter hunting, that's a real criterion. An industry standard isn't a fault, but its absence is one.

The nearest you can watch in sharp focus. Often forgotten, it changes everything for anyone who watches butterflies or dragonflies. A pair that focuses under 2 metres is a treat for close-up nature.

A simple benchmark: under 2 metres, you pick out a settled butterfly; between 2 and 4 metres, it's still fine; beyond that, forget the insects. It's free to check on the spec sheet, so you may as well look at it for your activity.