8×42 or 10×42?

Two numbers describe a binocular: magnification and objective diameter. Everything about low-light behaviour falls out of the relationship between them, and the relationship is not intuitive.

Exit pupil

Divide the objective diameter by the magnification and you get the exit pupil in millimetres — the width of the light cone leaving the eyepiece and entering your eye.

Here is why it matters. If the exit pupil is larger than your own dilated pupil, the surplus light hits your iris and is wasted. If it is smaller, your pupil is not being filled and the image looks dimmer than the binocular is capable of delivering.

A young, fully dark-adapted human pupil dilates to roughly 7mm. That capacity declines steadily with age — by middle age many people are closer to 5mm, and less again later. This is the reason a 7×50 "night glass" was standard marine issue for young sailors and is largely wasted on a sixty-year-old, whose iris cannot open wide enough to use the light on offer.

At dusk, an 8×42 delivering 5.25mm is filling most people's pupil. A 10×42 delivering 4.2mm is not — the image is measurably dimmer.

Twilight factor, which says the opposite

Twilight factor is the square root of magnification multiplied by objective diameter. For 8×42 that's √336 ≈ 18.3. For 10×42 it's √420 ≈ 20.5.

By that measure the 10×42 is better in low light — the reverse of what exit pupil suggests. Both figures are correct, because they measure different things.

Exit pupil predicts perceived brightness. Twilight factor attempts to predict detail resolution in dim conditions, on the reasoning that extra magnification helps you resolve detail even as the image dims. Magnifying a dim image can reveal detail that brightness alone won't.

Things neither number captures

• Coatings. "Fully multi-coated" means every air-to-glass surface has multiple layers. "Multi-coated" or "coated" means considerably less. This is one of the highest-value specifications on the box.
• Prism type and treatment. Roof prisms need phase-correction coating to avoid losing resolution and contrast; without it, the instrument underperforms regardless of other specs. Roof prisms also benefit from high-reflectivity dielectric mirror coatings. Porro prisms don't need either and can deliver excellent performance cheaply, at the cost of a bulkier body.
• Glass type. ED or HD elements reduce chromatic aberration — the purple fringing on high-contrast edges, most visible glassing a dark animal against bright sky.
• Hand shake. Above 10×, your pulse becomes visible in the image. This is a real limit on usable detail that no optical spec addresses.

Choosing

• Timber, close cover, moving fast: 8×42. Wider field of view, steadier image, brighter at dusk, easier to pick up a moving animal.
• Open country, glassing distant hillsides: 10×42. The extra magnification earns its keep when you are picking apart terrain at a mile.
• Over about 45 years old, hunting last light: lean 8×42. Your iris probably cannot use more than about 5mm of exit pupil anyway, and the steadier image helps more than the magnification.
• Buying 12× or 15×: budget for a tripod adapter in the same order. Handheld, you will not resolve the detail you paid for.
• Weight-limited mountain hunting: a quality 8×32 or 10×32 is a real option. You lose some dusk performance and save meaningful grams — and the binocular you actually carry beats the one in the truck.

Two practical points

Eye relief matters if you wear glasses. Below roughly 15mm you will not see the full field of view through spectacles. Twist-up eyecups with defined stops are a sign of a well-made instrument.

Buy a chest harness with the binocular. It keeps the glass still while you walk, eliminates the neck fatigue that makes people leave binoculars in the pack, and costs a fraction of the instrument. It is the highest-return accessory in the category.

FAQ

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Our Binocular buying guide covers the specs to compare and how to read them. Price classes, not live figures.