Choosing your first binoculars

Every pair of binoculars is described by two numbers, such as 8×42. The first is magnification: an 8× pair makes something eighty metres away look as it would at ten. The second is the diameter in millimetres of the front (objective) lenses, which determines how much light the instrument gathers.

That second number is the one people ignore and the one that matters most in practice, because birds are most active at dawn and dusk and much birdwatching happens under trees. A larger objective is brighter; it is also heavier, and weight is what stops you carrying binoculars.

The derived figure worth knowing:

Exit pupil

objective ÷ magnification8×42 gives 5.25mm; 10×42 gives 4.2mm. A human pupil dilates to roughly 7mm when young and rather less with age, so an exit pupil below about 4mm is visibly dimmer in poor light.

Field of view

Usually quoted in metres at 1000mHigher magnification narrows it. A wide field makes finding a moving bird dramatically easier, which is the main practical argument for 8× over 10×.

Close focus

The nearest distance at which they focusIrrelevant for birds at distance; important if you also want to look at butterflies and dragonflies, where 2m or better is worth having.

8×42, around £100–£200, roof prism, waterproof. That is the answer for the large majority of first-time buyers, and the reasoning is short.

8× rather than 10× because hand shake is magnified along with the image. At 10× a visible tremor obscures exactly the fine detail — an eye ring, a wing bar — that the extra magnification was supposed to reveal. Experienced birders overwhelmingly use 8×.

42mm rather than 32mm because the brightness difference at dawn, at dusk and under a canopy is large and obvious, and those are the productive times. 32mm is a reasonable choice if weight is a genuine constraint.

Around £100 because that is where the optics stop being the limiting factor. Below about £60, chromatic aberration, soft edges and poor coatings produce a dim, fringed image that causes eye strain. This is not audiophile territory: the difference between £40 and £120 binoculars is immediately visible to anybody.

• Zoom binoculars — Marketed as "8–24×". The optical compromises required to zoom are severe, the image is dim and soft throughout the range, and the mechanism introduces play. Avoid without exception.
• Ruby or red coatings — A visible red sheen on the objective lens is a cosmetic marketing feature that removes light from the image. Good coatings look faintly green or purple.
• "Military" or "tactical" branding — Means nothing.
• Very high magnification — 12× and above cannot be held steady by hand. They exist for tripod use.
• Porro vs roof prism, mostly — Porro prisms give better value optics per pound; roof prisms are more compact and easier to weatherproof. Both are fine, and at a given price porro is usually optically better.

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   Step 1: Set the interpupillary distance

   Fold the two barrels until you see one circular image rather than two overlapping ones. If you cannot get a single clean circle, the pair is wrong for your face.

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   Step 2: Set the dioptre

   Almost every pair has an adjustment on one eyepiece to correct for the difference between your eyes. Close the adjustable side, focus the other with the main wheel; then close the other and focus using the dioptre only. Most people never do this and wonder why nothing is quite sharp.

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   Step 3: Look at something with fine detail at distance

   Brickwork, a distant sign, bare twigs. Check the centre is genuinely sharp and then look at the edges of the field.

4. 4

   Step 4: Look at a high-contrast edge

   A dark branch against bright sky. Coloured fringing — purple or green — is chromatic aberration, and cheap binoculars show a lot of it.

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   Step 5: Check the weight around your neck

   The best binoculars are the ones you actually carry. Anything over about 800g becomes a decision every time you leave the house.