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Lighting and electronics

Eye relief

Also called long eye relief

What is eye relief on binoculars?

Eye relief is how far behind a binocular's eyepiece your eye can sit and still see the whole field of view, measured in millimeters. Glasses hold your eye farther back, so hikers who wear them need about 15mm or more to avoid a view ringed in black.

Eye relief at a glance

Glasses wearers
15mm minimum, 16 to 18mm comfortable
No glasses
Almost any figure works
Eyecups
Up without glasses, down with them

Why does eye relief matter for glasses wearers?

A binocular is designed to put the whole image at one spot behind the eyepiece. Without glasses, you press the eyecup to your brow and your eye lands there easily. Glasses add 10mm or more of distance, and if the eye relief is shorter than the space they take up, you only see the middle of the view, with a dark ring around it. That's why eyeglass wearers are told to look for about 15mm or more.

How do you use the eyecups?

Almost every modern binocular has eyecups that twist or fold. Leave them fully up if you don't wear glasses, so your eye sits at the right distance. Twist them all the way down if you do, so your lenses can get close to the eyepiece. If you still see a black ring with the cups down, the eye relief is too short for your frames.

How much eye relief do binoculars have?

Most good 8x binoculars publish between 15 and 18mm, and higher magnifications tend to have less. Some cheaper pocket models run much shorter; one popular 8x25 lists 10mm, which is comfortable only without glasses. Thick or wraparound frames sit farther from the eye, so 17 or 18mm is the safer number for them.

What this means when you're choosing

  • At least 15mm of eye relief if you hike in glasses
  • Twist-up eyecups with firm click stops that stay where you set them
  • More margin, around 17 or 18mm, for thick or wraparound sunglasses

The common mistake

Leaving the eyecups up while wearing glasses

With the cups extended, your glasses hold your eye too far back and even a long eye relief binocular shows a shrunken, vignetted view. Twist them down first and judge the view after.

Common questions

Can you use binoculars with glasses on?

Yes. Twist the eyecups down and choose a binocular with about 15mm or more of eye relief, and you'll see the full view through your glasses.

Is longer eye relief always better?

Not quite. Without glasses, a very long eye relief can make it harder to hold your eye in the right spot, which is what the adjustable eyecups are for.

Where this is defined

Where this comes up

Guides on Tom's Hiking where eye relief changes which product is right for you.

More in lighting and electronics

Lumens

Lumens measure the total light a headlamp emits in all directions. It's the headline number on every box and it says nothing about where that light goes, so a 400 lumen lamp with a bad beam can be less useful than a 200 lumen one with a good one.

Beam distance

Beam distance is how far a headlamp usefully projects light, quoted in meters. The industry standard measures it to the point where illumination falls to 0.25 lux, roughly the brightness of a full moon, which is dimmer than most people picture.

Flood vs spot beam

A flood beam spreads light wide and close for camp tasks and slow ground. A spot beam concentrates it into a narrow cone that reaches further, for seeing ahead at speed. Most trail headlamps mix both, and the mix matters more than the lumen count.

Regulated output

A regulated headlamp holds a steady brightness as its battery drains, then drops off sharply near the end. An unregulated one fades continuously from the moment you switch it on, so it's dim long before the battery is actually empty.

Reactive lighting

Reactive lighting uses a sensor to measure reflected light and adjusts brightness automatically, dimming when you look at a map and brightening when you look down the trail. It extends battery life and can feel unpredictable at speed.

IP rating

An IP rating states how well a device resists solids and water, under the IEC 60529 standard. The first digit covers dust and the second water, so IPX4 is splash resistant, IPX7 survives brief submersion, and IP67 does both.

GPX file

A GPX file is the standard open format for storing GPS routes, tracks and waypoints as plain text. It's what you download from a trail site and load onto a watch or phone, and almost every mapping app and GPS device reads it.

Breadcrumb navigation

Breadcrumb navigation draws your loaded route as a line on a blank screen with an arrow for your position, without any underlying map. It tells you whether you're on the route, and nothing about what's around you.

ABC sensor

ABC stands for altimeter, barometer and compass, the three sensors in an outdoor watch. The altimeter and barometer are the same pressure sensor read two ways, and the compass is a magnetometer that gives you a heading when you're standing still.

Exit pupil

Exit pupil is the width of the beam of light a binocular sends to your eye, found by dividing the objective lens size by the magnification. An 8x25 has a 3.1mm exit pupil and an 8x32 has 4mm; the bigger number looks brighter in dim light.

ED glass

ED glass, short for extra-low dispersion, is lens glass that bends red, green and blue light by nearly the same amount. In binoculars it cuts the purple and green fringes around dark edges against a bright sky, so the image looks sharper and truer in color.

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