Extra Low Dispersion Glass (ED Glass)

In recent years we have seen a huge increase in the amount of binoculars that use ED Glass (Extra Low Dispersion Glass) in their lenses. What used to be found only on a few of the very top end “super” binoculars, is now quite common and is used on a whole range of binoculars, even fairly low cost ones.

So with so many manufacturers using ED Glass, do you know what it rally means, why it is used and how does it help improve the view through your optics? Well if not, I will try and answer those questions in this article, but it can get a little technical, so hold onto your hats, but stick with me as it is kind of cool and I will try to explain it to the best of my knowledge and as simply as I can:

Dispersion in Optics

To get started and before we can talk about how one type of glass is better than another, lets first figure out what dispersion is and what it means in the world of optics:

As most people, especially the Pink Floyd fans among us will know, if you pass light through a prism, it will break white light up into its component colors. The blue end of the spectrum is bent the most, and the red end is bent the least.

So the prism spreads or disperses out the colors that make up white light and dispersion is a simply a measure of how much the colors are spread out.

Dispersion in Lenses

The lenses on your binoculars are used to focus the light they capture onto a single point. However like the prism, a small amount of dispersion occurs as the light passes through the lens and the different wavelengths of light are focused to slightly different positions. This distortion is known as chromatic aberration and is seen as fringes of colour around the image produced and is known as colour fringing.

Colour Fringing caused by Chromatic Aberration

The image above demonstrates the color fringing around the edges of the flower, seen when looking at it through a lens with chromatic aberration.

Reducing the Dispersion

There are certain situations when it is preferable to split the wavelengths of light, but obviously this color fringing in not ideal on a pair of binoculars and so being able to gather up all these different wavelengths of light and focus them on a single point is what you want your objective lenses to achieve.

So it stands to reason that by Reducing the Dispersion – it will reduce the amount of chromatic aberration – which in turn will reduce the amount of color fringing around the image.

There are a number of ways that this can be done and sometimes as you will see below, a combination of some of them produces the best results:

Compound Lenses
A single or simple lens that consists of a single optical element cannot compensate for chromatic aberration, so compound lenses are used which are made up of a number of lens elements that have different optical characteristics and therefore disperse the wavelengths of light differently. These are then bonded together to form a single lens and which, if designed very cleverly can help to correct this aberration.

Achromatic Doublet lensAchromatic Doublet
The most common type of compound lens used to reduce chromatic aberration in the objective lenses of binoculars has two lens elements with differing dispersion properties and is known as an achromatic doublet (or achromat). A typical “standard glass” achromat is made of a low dispersion crown glass and a high dispersion flint glass.

However, whilst an achromatic doublet is a big step forward when compared to a simple lens as it reduces the amount of chromatic aberration over a certain range of wavelengths, they do not produce perfect correction.

Apochromatic Lens (APO)
Apochromatic Lens (APO)An apochromat, or apochromatic lens or lens system usually consists of three elements and brings light of three different frequencies to a common focus. These can produce even better correction of chromatic aberration, combined with improved correction of spherical aberration than the achromatic doublet can.

For more take a look at my complete guide to Apochromatic (APO) Lenses in Binoculars.

An example of a binocular that uses a Apochromatic Lens with ED Glass elements is the excellent Vortex 8x42 Razor HD Binoculars that produce one of the best images I have ever seen.

Extra Low Dispersion Glass (ED Glass)

Lens Materials
However this is not where the story ends as different lens materials such as specialised coatings or lenses made from different types of glass may also be used to minimise chromatic aberrations.

As we have seen, an achromatic lens made with conventional glass materials can match focal lengths of two different wavelengths, so red and blue colors at both ends of the wavelengths of visible light for example. The chromatic aberration can be reduced to a certain extent by conforming their focal lengths. However, because light with other wavelengths such as green has different focal lengths, you still get some residual chromatic aberration which is known as secondary spectrum.

The problem of this secondary spectrum cannot be solved with any combination using only conventional glass, but particular optical materials which have a unique characteristic of dispersion can. I think this characteristic is known as anomalous partial dispersion (I may be wrong here – if someone knows for sure please let me know). ED (Extra-low Dispersion) glass has this unique characteristic and when combined with other glasses it minimizes the effects of the secondary spectrum and when you compare this with an achromatic lenses made with standard glass, ED glass reduces chromatic aberration and therefore the color fringing to a remarkable degree.

Such glass types are often more expensive and are thus used sparingly unless needed. Moreover, use of such special glass types often requires a careful choreographing with other elements of special type, to balance the overall behavior.

So to summarize:
Extra low dispersion glass prevents chromatic aberration because it gives the designer of a multi-element objective lens more options to concentrate and direct the wavelengths of light more effectively, thus they can control and minimise aberrations, in particular, chromatic aberration. Generally speaking, the better the aberrations are controlled the cleaner and brighter the image will appear.

Consequently most higher end optics now come equipped with extra low dispersion glass lenses. Camera’s with the glass tend to take pictures that are clearer and sharper with little or no chromatic aberration and binoculars and telescopes transmit clearer and sharper images to your eyes.

Some other features of ED Glass:

  • ED glass is less stable with temperature than conventional glass, and so the focal lengths of these lenses change slightly with temperature
  • ED glass also has a low index of refraction so it requires more deeply curved elements for the same focal length

ED Glass (Extra Low Dispersion) vs Standard Glass
The illustration above is one that Vanguard Sport Optics use to demonstrate the difference between using standard and extra low dispersion glass (ED glass)

Highly Rated ED Binoculars

Below are some binoculars that use Extra Low Dispersion Glass (ED Glass) in their lenses and which I rated very highly when I reviewed them:

High End ED Binoculars

Vortex 8x42 Razor HD Binoculars

Vortex 8 x 42 Razor HD BinocularsThese Vortex binoculars use an apochromatic lens design that includes high density ED glass elements and the use of the very best quality coatings like dielectric and phase correction coatings on the roof prisms that all join together and produce an extremely high quality and bright image and one of the best I have seen.

These Vortex binoculars ooze quality and the water and fogproof body is nice and light as well as strong thanks to a magnesium chassis. I also really like the exterior design that is both comfortable and secure to hold.

Price: (5/6) High Value Binoculars

Rated as one of the best value for money binoculars by Best Binocular Reviews

Best Binocular Reviews Ratings:
Body Quality: 9/10 90%
The VERY Best
Body Stats: 9/10
Optical Quality: 10/10
Optical Stats: 8/10
Image Quality 9/10
Extras & Details: 9/10

Mid – High End ED Binoculars

Vanguard 10x42 Endeavor ED Binoculars

Vanguard 10x42 Endeavor ED BinocularsVanguard`s top-of-the-line Endeavor ED binoculars features include an ergonomic, open-bridge design, a wide view angle, BaK4 roof prisms, anti-reflection coatings end ED glass that all help to deliver exceptional light transmission to produce a excellent quality image.

The body is Nitrogen-charged and o-ring sealed making them both waterproof and fogproof to take on the most challenging weather conditions.

Price: (4/6) Mid – High Value Binoculars

Rated as one of the best value for money binoculars by Best Binocular Reviews

Best Binocular Reviews Ratings:
Body Quality: 9/10 87%
Outstanding
Body Stats: 8/10
Optical Quality: 9/10
Optical Stats: 9/10
Image Quality 9/10
Extras & Details: 8/10
 

Eagle Optics 8x42 Ranger ED Binoculars

Eagle Optics 8x42 Ranger ED BinocularsThese impressive Eagle Optics binoculars are packed with quality features like ED glass & phase corrected Bak-4 prisms combined with a very comfortable and functional open bridge body design that is both water and fogproof.

They also have a very wide field of view, which should really appeal to birders.

Price: (4/6) Mid – High Value Binoculars

Rated as one of the best value for money binoculars by Best Binocular Reviews

Best Binocular Reviews Ratings:
Body Quality: 8/10 85%
Outstanding
Body Stats: 8/10
Optical Quality: 10/10
Optical Stats: 9/10
Image Quality 9/10
Extras & Details: 7/10

Mid Price ED Binoculars

Carson 10x42 3D Series BinocularsCarson 10x42 3D Series Binoculars

A tough but very lightweight and waterproof body protects some really high quality optical components including the use of ED glass and phase-coated BaK-4 roof prisms that produce a bright and high quality image.

Extras like an included binocular harness and the fantastic wrap around BinoArmor carry case add extra value to this already excellent pair of optics.

Price: (3/6) Mid Priced Binoculars

Best Binocular Reviews Ratings:
Body Quality: 7/10 78%
Excellent
Body Stats: 8/10
Optical Quality: 8/10
Optical Stats: 8/10
Image Quality 8/10
Extras & Details: 8/10
 

Vanguard 10x42 Spirit ED Binoculars

Vanguard 10x42 Spirit ED BinocularsA tough but very lightweight and waterproof body protects some really high quality optical components including the use of ED glass and phase-coated BaK-4 roof prisms that produce a bright and high quality image.

Extras like an included binocular harness and the fantastic wrap around BinoArmor carry case add extra value to this already excellent pair of optics.

Price: (3/6) Mid Priced Binoculars

Rated as one of the best value for money binoculars by Best Binocular Reviews

Best Binocular Reviews Ratings:
Body Quality: 8/10 87%
Outstanding
Body Stats: 9/10
Optical Quality: 9/10
Optical Stats: 8/10
Image Quality 8/10
Extras & Details: 9/10


Further reading

You can also take a look at:

Where to Buy ED Binoculars


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