Compact Binoculars

Compact binoculars, small enough to be carried and therefore used

Compact binoculars are the pair that ends up in a coat pocket rather than in a cupboard, and the range here runs from pocket-sized roof prism models to slightly larger waterproof designs sized for hiking, birds and wildlife. The argument for them is behavioural rather than optical: a large instrument gathers more light, but one left at home gathers none. The question is how much performance you will trade for the bulk you actually carry.

What makes a pair compact?

The front lens, mostly. Objective diameter is what sets the physical size of the barrels, so a small objective is what allows the folding, pocketable shape. The prism design does the rest: a roof prism runs straight through the barrel and gives a slim, tubular body, while the older porro layout steps sideways and produces the wide, angled shape that will not fold flat. Almost everything sold as compact uses roof prisms and a hinge that closes the barrels together.

What do you give up by going small?

Light, and steadiness at the eyepiece. A small objective feeds a narrow beam to the eye, which is fine in daylight and noticeably dimmer at dawn, dusk and under woodland cover. Full-size instruments such as 8x42 binoculars exist precisely because a larger front lens holds the image bright as the light drops. A compact pair also has less body mass to damp out hand tremor, so the view moves more. Neither is a fault, both are the cost of the size.

Which magnification can you hold steady?

Lower than most people expect. Every bit of magnification enlarges hand movement as well as the subject, and in a light body that becomes obvious quickly. Eight times is comfortable held freehand for long periods. Ten is usable braced against a fence post or a rucksack and tiring without support. Anything higher needs a tripod to be worth the reach, which is why 10x42 binoculars in a full-size body feel steadier than a lighter pair at the same power. If one hand has to stay free, a monocular is the more honest solution.

What matters beyond magnification on a compact pair?

• Eye relief: the distance the eye can sit back and still see the full field. Spectacle wearers need a generous figure and eyecups that twist or fold down.
• Close focus: the shortest focusing distance, which decides whether butterflies and garden birds are usable subjects.
• Sealing: an O-ring sealed, nitrogen-filled body keeps damp air out of the barrels and stops internal fogging on a cold morning.
• Hinge stiffness: the hinge sets the spacing between your eyes. Too loose and it shifts in a pocket, too stiff and it fights adjustment with gloves on.

Do they work with glasses on?

They can, provided the eyecups fold or twist down and the eye relief is long enough to clear a lens. Without that, the view narrows to a small circle with a black ring around it. Long-sighted and short-sighted wearers can often take the glasses off and use the dioptre adjustment on the right barrel to compensate, but that fails for astigmatism, which no focusing wheel corrects. Set the dioptre once, on a distant fixed object, and leave it alone afterwards.