How to use binoculars: eyecups, eye spacing and diopter

Set three things before the first look

Nikon's manual page for CF binoculars gives one order: retract the eyecups if you wear eyeglasses, move the tubes until the two fields form one circle, then set the diopter. Every one of the 554 models in the best binoculars compared list has these controls in some form.

The three settings fit your face and your eyes, so you set them once. After that, only the center focus wheel changes from one target to the next.

The steps, in the order makers give

1. Eyecups. For eyeglasses, Nikon says to turn a turn-and-slide eyecup clockwise to the fully retracted position, or fold back rubber eyecups. ZEISS says the same: eyecups go in for glasses.
2. Eye spacing. Celestron says to move the two tubes toward or away from each other. Nikon says to move them down or up until the left and right fields align as one circle.
3. Diopter, left side. Set the diopter dial to its zero mark, cover the right objective lens and pick a distant, still target. Focus the left eyepiece with the center dial, as Celestron lists it.
4. Diopter, right side. Cover the left objective lens and turn the diopter dial until the right side is sharp. Celestron then has you uncover the left side and fine-tune with the center dial.
5. Zoom models. Nikon says to set maximum magnification before you set the diopter.

Eyecups and eyeglasses

Nikon defines eye relief as the distance from the outer surface of the eyepiece lens to the position where the exit pupil forms. It recommends longer eye relief for eyeglass wearers, without giving a figure.

In the catalog, the median eye relief is 16.4 mm, and 337 of 529 models with a printed figure list 16 mm or more. If you wear glasses, fold or twist the eyecups in and check that the full circle of view still shows. The binoculars for eyeglass wearers list sorts models by this figure.

Eye spacing: match the tubes to your eyes

Swarovski Optik defines interpupillary distance as the distance in millimeters between the centers of your two pupils. Celestron notes that it differs from person to person, and that low-power models with small lenses can usually be set to smaller distances.

Makers print the adjustable range. Vortex lists 60 to 76 mm for its Crossfire HD 10x50. Celestron adds that some binoculars carry a scale on the center pivot, so you can note your setting and return to it.

The diopter ring and different eyes

ZEISS says diopter compensation corrects different prescriptions in the left and right eyes, and that the range should ideally be at least 4 D either way. Swarovski Optik adds that it is not needed if both eyes see equally well.

Vortex says the ring usually sits directly under the right-side eyecup. If yours is on the left eyepiece, Nikon says to reverse the order: focus the right side with the center wheel, then turn the ring for the left side. Set it once and leave it.

Holding binoculars steady

National Audubon says binoculars that magnify an image eight times also magnify the small movements of your hand eight times. That is why higher power tires you.

ZEISS puts free-hand binoculars at 7 to 10x. Celestron says a tripod is not required at 8x to 12x but becomes necessary at 15x and higher. The British Astronomical Association says more than 10x is difficult to hold and should be mounted. The RSPB calls 8x or 10x the best compromise for birdwatching. The guide to when binoculars need a tripod covers the rest.

How the size changes what you see on first use

A wide view makes a target easier to find. The 8x42 has a median field of view of 393 ft and an exit pupil of 5.3 mm, across 75 models. The 12x50 has a median of 277 ft across 36 models, so the same target sits closer to the edge of the view.

The 10x42 sits between them at 336 ft, with 87 models. Weight matters on a long carry: the median 8x32 weighs 18.5 oz against 29.2 oz for the 12x50. See the 8x42 vs 10x42 comparison or the 8x42 size page for the detail.