ED Glass. Why it makes a big difference for binoculars

How "better glass" makes the views through binoculars sharper and brighter.

You might have heard that ED binoculars or those with "ED glass" are well worth the extra costs as they offer better viewing.  We've had many folks ask us why this is and how they works. After all, we assume the glass in windows and cars is pretty much the same and the only to improve the views through them is simply to make sure our windows are as clean as possible! 

To a degree that true, however the glass used in the lenses in binoculars and telescopes has one major difference to that we find in windows. The glass used in those is flat and the glass in lenses is curved to focus the light from objects in the distance. This introduces a major problem: chromatic aberration.

Let's blame Sir Isaac Newton

Everyone has used a magnifying glass.

Just when experimenters discovered that curved glass lenses or crystals could change or magnify you're looking is not exactly known and probably goes back a few thousand years. Their curved shape changes the view seen through them.  The first telescope was invented in the early 17th century by combining more than one lens. (BTW, this is one of those discoveries that could have been make hundreds or possibly even thousands of years early. It simply that nobody fiddled around with lenses enough until then.)

Sir Isaac Newtown demonstrating how light is split in a coloured spectrum and thus increasing the price of performance binoculars for all time.

We also all know that Sir Isaac Newton discovered that a glass prism can split visible light into different wavelengths which make a rainbow. While this is an important piece of knowledge, it's also the reason that telescopes and binoculars can have problems with slightly blurry or colour-fringed images. 

While light travels through the middle of a lens pretty much straight through. As the lens curves towards the edges, its cross section shape resembles a prism. It will bend or "refract" the light to form an image. It also splits different colours just as a prism does and these different colours are focussed at slightly different positions.**

This is called chromatic aberration. The diagram above will give you a rough idea of how it works, although it is greatly exaggerated. However, even small amounts of difference in the focus of various colours are enough to produce marked colour distortions and fringing around objects. 

The first fix for chromatic aberration

Opticians get around the a large part of the problem of chromatic aberration through the addition of a secondary lens* which has a different shape.

Telescopes and binoculars like these are said to have achromatic lenses. This design gets rids of most of the colour distortions but not quite all. 

An illustration from Nikon showing the effects of chromatic aberration

The impact of the remaining small bits of chromatic aberration, sometime called secondary spectrum, isn't major but it's still there. In many cases, two element lenses using the same type of glass will produce images that are pleasing enough for many. 

ED Glass for further improvements 

For even better quality viewing, one of the lens elements uses a different type of glass. This is denser compared to the other element and will bend or disperse the light just a little more. This is called Extra Dispersion or abbreviated to ED. 

A combination of two different types of carefully selected glass elements will even further the effects of chromatic aberration. The downside is that these types of glass lenses costs more to produce and this will bump up the cost of the binoculars, often by a fair amount.

Why are we so fussed about getting rid of the colour fringing from chromatic aberration?

Our minds do a lot of processing of the images that our eyes see to produce what out minds perceive. 

First of all, colour fringing blurs and distorts what's viewed through binoculars. Our efforts to compensate for this can stop us seeing the finer details of the view. This is important when trying to distinguish on birds, especially when viewing under less than ideal conditions or when they're in the distance. 

The effects of chromatic aberration are most noticeable around the edge of the field of view. Locating birds needs as wide as possible viewing.

 Can I get by with non ED binoculars?

Yes, most certainly! Under most circumstances they'll do fine. It depends on your budget and how critical the quality of your binocular optics are to you.

For example, the Nikon Prostaff P7 8x42  are some of the most popular and affordable quality binoculars with our BINTEL customers.

Do the terms ED and HD mean the same thing?

Its hard to say. Many binoculars makers use the term "HD" of high definition to suggest quality optics.*** This may be the case. It's more of a marketing term than a technical description of what's inside them. There's even some that are described as both HD and ED mainly as many folks don't really know what ED relates too.

Generally speaking, binoculars that have the term ED in their product specifications will have extra dispersion optics. They cost a little more and offer better viewing.

Bottom line:

ED or ED glass is a term used to show binoculars have higher quality lenses. They help avoid the worst effects of chromatic aberration that's found in all glass lenses. 

Binoculars with ED optics will cost more, but for many uses such as bird watching are well worth the extra cost. 

As always, have a talk with the team at BINTEL to help select the best binoculars for your next bird watching adventure!

Cheers,

Earl White 

BINTEL 

5th September 2025

* It's not uncommon for binoculars to have three or even more glass elements to further improve the colour performance and offer increased contrast. One of the terms used for these are apochromatic or APO lenses.

** Isaac Newtown realised that the splitting of light into different colours was an issue with early telescopes and instead of spending too long trying to solve the problem, he simply invented an entirely new type of telescope that used a curved mirror instead of a lens; the Newtonian Reflector. This design was so good, astronomers still use them, and we have the same type of telescope on sale at BINTEL to this day.

*** There many other ways to improve contrast and image quality in binoculars. We'll cover those in future articles.