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Lighting and electronics

ED glass

Also called extra-low dispersion glass, low-dispersion glass, HD glass

What is ED glass in binoculars?

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.

ED glass at a glance

Stands for
Extra-low dispersion
Fixes
Color fringing, or chromatic aberration
Shows most
Dark edges against a bright sky

What problem does ED glass solve?

Ordinary glass splits white light slightly, bending blue more than red, so the colors of an image don't all focus at the same point. The result is chromatic aberration: a purple or green halo around a branch, a bird or a ridgeline against a bright sky. ED glass has lower dispersion, so the colors land closer together and the fringes mostly disappear.

Is ED glass worth it for hiking?

Often, yes, because hikers look at exactly the scenes where fringing shows: dark trees and rock against a bright sky. It also makes the image look crisper in general. It isn't everything, though. Coatings, prism quality and build matter too, and a well-made binocular without ED glass can beat a poorly made one with it. Some makers sell HD glass instead, which may or may not be a true ED formula, so treat the label as a hint and read reviews for fringing.

What this means when you're choosing

  • ED named in the maker's specs, not only in a marketing headline
  • Reviews that mention fringing on high-contrast edges
  • Good coatings alongside it, which matter as much for brightness

The common mistake

Treating ED as a guarantee of a great binocular

ED glass fixes one kind of error. A binocular can have it and still be let down by weak coatings, a narrow field of view or short eye relief, so judge the whole spec sheet.

Common questions

What is the difference between ED and HD binoculars?

ED names a type of glass. HD is a marketing term makers use for their better optics, and it may or may not include true ED glass.

Do cheap binoculars have ED glass?

Some do now, even around the middle of the market. The glass alone doesn't make a binocular good, so look at coatings, eye relief and reviews too.

Where this is defined

Where this comes up

Guides on Tom's Hiking where ed glass 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.

Eye relief

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.

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