Chromatic Aberration and Colour Fringing in Binoculars

Chromatic Aberration in Binoculars - Optics Central

Chromatic Aberration. Colour Fringing. ED Glass and HD Glass. These are all terms that we have heard before, but how do they affect your binoculars? What do they actually mean, and how should they influence which pair of binoculars you choose?

A Short Introduction to Chromatic Aberration and Colour Fringing in Binoculars

Chromatic Aberration in Binoculars

Light seen through your eyes (visible light) has a range of different wavelengths. To ensure images remain as sharp as possible, binocular lenses aim to gather and refract all these different wavelengths of colour to the same point.

Light with longer wavelengths (colours such as orange or red) are not refracted as strongly as light with shorter wavelengths (colours such as blue or violet).

Chromatic Aberration happens when the lenses in your binoculars fail to refract all wavelengths of colour to the same point. It is caused by lens dispersion, where different colours of light pass through a lens at different speeds (Mansurov, 2019).

Diagram of Chromatic Aberration
Figure 1. Diagram of Chromatic Aberration (Mellish, 2006).

Colour Fringing in Binoculars

Chromatic aberration results in Colour Fringing, where some binoculars may present images that are not as sharp as others. Instead, they might display a purple-green hue, especially around the edges of a subject.

Take a look at Figure 2 to see what colour fringing can look like.

Purple Colour Fringing
Figure 2. Purple Colour Fringing (Kitkatcrazy, 2007).

How Do We Reduce Chromatic Aberration?

Colour fringing and chromatic aberration in binoculars can be improved by reducing lens dispersion. There are several ways to do this, varying in cost and effectiveness.

Compound Lenses

Compound Lenses are the simplest method to correct chromatic aberration. They are created with multiple lens elements, all with different optical properties that refract wavelengths of light differently.

Figure 3 shows you how a compound lens is designed.

Chromatic Aberration Diagram
Figure 3. Diagram of an Achromatic Doublet Lens (Mellish, 2010).

The type of compound lenses we see most often are Achromatic Doublet. They use two lens elements with different optical characteristics. Conversely, Apochromatic (APO) lenses normally consist of three lens elements to bring three wavelengths of light to a focal point. Although APO lenses are more effective against chromatic aberration, achromatic doublets are more common as they are cheaper, lighter and more compact (Best Binocular Reviews, n.d.).

ED Glass

Although standard compound lenses reduce the extent of chromatic aberration for colours such as red and blue, there are still wavelengths of light (such as green) that compound lenses do not conform to. This results in residual chromatic aberration, known as the secondary spectrum (Nikon, n.d.).

This is where Extra-Low Dispersion Glass (ED Glass) comes in. ED glass uses specially formulated coatings with rare-earth compounds to minimise chromatic aberration. The specific formulation of extra-low dispersion glass means that, unlike standard glass, it can reduce the size of the secondary spectrum. ED glass can also be combined with compound lenses to near eliminate chromatic aberration (Celestron, n.d.).

For most higher-end binoculars and optical instruments, it is imperative that ED glass is used. Some examples of popular ED Binoculars include:

ED Glass vs HD Glass in Binoculars

Whilst shopping for binoculars, you might have seen the terms “HD Glass”, “High Definition Glass” or “High-Density Glass” used before.

A common misconception is that ED and HD glass mean the same thing. But depending on the manufacturer, HD Glass can carry a multitude of meanings. This is where the confusion arises.

Generally speaking, most manufacturers use the term “HD” to signify that the binoculars in question incorporate high-end technology that inexpensive binoculars might not. The particular set of high-end technologies may differ between brands.

For example, the Vortex Diamondback HD carries the “HD” in its name to signify improvements to the quality of its optics and chassis with dielectric and ArmorTek lens coatings. But unlike the higher-end Vortex Viper HD, the Diamondback does not feature ED Glass.

Additionally, “HD” may not be the only term a brand uses to determine high-end technology. Take Kowa for example – they use the term XD to represent extra-low dispersion (ED) glass (Kowa Prominar, n.d.).

To minimise chromatic aberration, ensure you understand what HD actually means before buying a pair of binoculars.

Which Pair of Binoculars Should You Get?

So, is chromatic aberration and colour fringing in binoculars something you want to consider? It really depends on what you are looking for in a pair of binoculars. For a near-perfect viewing experience, invest in a pair with high quality, extra-low dispersion optics. If you are after a more budget-friendly option for casual viewing, a pair of HD, non-ED binoculars might be the better option.

It comes down to how much chromatic aberration you are willing to deal with and how much you are willing to spend.

But don’t forget, chromatic aberration is only one consideration in a list of factors you need to pay attention to when purchasing binoculars. Factors such as lens coating types, prism types, eye relief and focusing types should also be considered, for example.

And if this all sounds confusing, we also have a comprehensive guide to binoculars published on our website.

No matter what you choose, we hope you learn to love your pair of binoculars.

As always, happy sightseeing!

REFERENCES

Best Binocular Reviews. (n.d.). Apochromatic (APO) Lenses in Binoculars. Best Binoculars & Binocular Reviews Website. https://www.bestbinocularsreviews.com/blog/apochromatic-apo-lenses-in-binoculars-03/

Celestron. (n.d.). ED Glass. https://www.celestron.com/pages/ed-glass#:~:text=ED%20stands%20for%20%22extra%2Dlow,visual%20defect%20called%20chromatic%20aberration

Celestron. (2018, April 17). What are the differences between the optical coatings? https://www.celestron.com/blogs/knowledgebase/what-are-the-differences-between-the-optical-coatings

Kitkatcrazy. (2007). Purple fringing. [Online image]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Purple_fringing.jpg

Kowa Prominar. (n.d.). http://kowa-prominar.com/concept/technology.html

Malacara, D. (2011). Color Vision and Colorimetry: Theory and Applications, Second Edition. SPIE Press.

Mansurov, N. (2019, December 10). What is Chromatic Aberration? Photography Life. https://photographylife.com/what-is-chromatic-aberration

Mellish, B. (2006). Chromatic aberration lens diagram. [Online image]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Chromatic_aberration_lens_diagram.svg

Mellish, B. (2010). Lens6b-en. [Online image]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Lens6b-en.svg

Nikon. (n.d.). Coating on roof (Dach) prism. https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/technologies/technologies_07.htm

Nikon. (n.d.). Comparison of ED glass and ordinary lens. https://imaging.nikon.com/lineup/sportoptics/how_to/guide/fieldscopes/choosing/choosing_03.htm

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