Anti-Reflection Lens Coatings on Binoculars

Why are Lens Coatings Used?

Imagine walking through a thick forest. Sunlight struggles to penetrate the dense foliage, making it difficult to see. Similarly, light encountering and then trying to pass through a series of lenses in binoculars encounters obstacles. Here’s what happens:

  • Reflection: When light strikes an air-to-glass surface, a portion of it is reflected, just like light bouncing off a mirror. So because this reflected light doesn’t reach your eyes, it does not contribute to the image you see, effectively reducing the potential of the overall quality and brightness of the image.
  • Scatter: Light can also scatter within the glass itself, causing the image to be less sharp, so more hazy and blurry.

Special lens coatings are applied to help combat these issues and improve the overall viewing experience through binoculars.

So with binoculars, scopes, monoculars and many other optical devices like camera lenses, anti-reflection coatings are designed to assist with light transmission thus helping to produce a sharper, brighter image with improved contrast. These days almost all good quality binoculars have some sort of anti-reflection coatings applied to at least some of the air-to-glass surfaces of their lenses.

How Do Lens Coatings Work?

These anti-reflection coatings are often made up of a number of incredibly thin layers, typically measured in microns (one-thousandth of a millimeter), that are deposited on the lens surface. Each has a distinct index of refraction which alters the lens’s performance with different wavelengths and at different incident angles and work by manipulating light in specific ways:

So they kind of function like a tiny “traffic cop” for light. The coating is composed of alternating layers of materials with different refractive indices (how they bend light). These layers cause incoming light to interfere with the reflected light, cancelling it out and allowing more light to pass through to your eye. Imagine two sound waves perfectly out of phase, cancelling each other out – that’s the principle at work here!

Objective Lenses on the Swarovski NL Pure 8x32 Binoculars

Because of this, color effects often appear at oblique angles and so you can sometimes see these coatings as they are usually what produces the blue, red, or green reflections you see when you look into the front of a binocular’s objective lens at different angles.

Image Demonstarating relected light

Improved Transmittance and Image Brightness
In any lens, some of the light that passes through the lens is reflected by the front (incident light) and rear (exiting light) surfaces. This reflection reduces the amount of light passing right through the lens and thus to your eyes.

If the amount of light reflected away is very bad, the image you see when you look through the binoculars will be darker than on binoculars that transmit more light.

As well as producing a less bright image, reflected light may cause ghosting, flares and also affect the contrast of the image.

So during the manufacturing process, these special anti-reflective coatings are added to the lenses on better quality binoculars, which drastically decreases the amount of surface reflection losses and can significantly increase the transmittance of light to ensure you get as bright, clear and sharp image as possible.

Ghost Imaging
What is more, the undesirable secondary reflections that can interfere with the transmitted image, producing what is known as “ghost imaging” can also be all but completely be eradicated using good anti-reflection coatings.

More Benefits:
Because the amount of reflection from the lenses is reduced, these coatings also help in that they reduce the glint coming off the lenses. This is important if the user of the binoculars does not want to give away their position. Important for military, law enforcement, hunting and even general wildlife observation applications.

Coated vs Multi-Coated vs Fully Multi-Coated

Lens CoatingsWhen researching a binocular, it is important to note how the manufacturer describes their coatings:

Coated – means a single layer antireflection coating on some lens elements, usually the first and last elements (the only ones you can see).

Fully Coated – means that all air-to-glass surfaces are coated, which is obviously an improvement on a single-layer coating.

Multi-coated – means that at least some surfaces (again, usually the first and the last) have multiple layers of antireflection coatings. (A multilayer coating effectively reduces reflected light that cannot be eliminated with a single-layer coating, and increases the transmittance of light.) Multiple layers are about an order of magnitude more effective than a single layer.

Fully Multi-Coated – means that all air-to-glass surfaces have received multiple layers of anti-reflection coatings – this is what you want in your binoculars.

The table below shows Transmittance by type of coating:

  Per Single Lens Surface 10 Lens & Prism Surfaces
No Coating: 96% (0.96) x Power of 10 = 0.66 66%
Single-Layer: 98.5% (0.985) x Power of 10 = 0.86 86%
Multilayer Coating: 99.5% (0.995) x Power of 10 = 0.95 95%
     

Materials Used in Lens Coatings

The specific materials used in lens coatings are a closely guarded secret by manufacturers. However, common materials include:

  • Magnesium fluoride (MgF2): A popular choice for single-layer anti-reflective coatings due to its effectiveness and ease of application.
  • Silicon dioxide (SiO2): Used in multi-layer coatings for its excellent light transmission properties.
  • Tantalum pentoxide (Ta2O5): Known for its high refractive index, useful for creating the desired optical effects in multi-layer coatings.
  • Aluminum Oxide (Al2O3): Chosen for their specific refractive indices and transparency to visible light.

Application of Lens Coatings

Applying lens coatings is a complex and precise process, often involving multiple steps:

  1. Surface Cleaning: The lenses are meticulously cleaned to remove any dust, oil, or contaminants that could affect the coating adhesion.
  2. Vacuum Deposition: The lenses are placed in a vacuum chamber where the coating materials (often in the form of pellets) are vaporized and deposited onto the lens surface in a controlled manner. Different techniques like sputtering or evaporation can be used for this purpose. For more on this process, take a look at this article and video on How Binoculars are Made.
  3. Monitoring and Quality Control: The thickness and uniformity of the coating are precisely monitored throughout the process to ensure optimal performance.

Cost of Fully Multi-Coated Binoculars

There was a time when you would only find fully multi-coated binoculars at the top end of the market. But as production methods improved and because of the influence of the cheaper production costs from mostly Asian manufacturers, more and more binoculars were being fully multi-coated.

So because of this increased competition and better manufacturing techniques, most mid-range and even some low-cost binoculars come fully multi-coated with anti-reflection coatings. So now a bin needs far more optical features, like for example the use of ED Glass in their lenses, to even have a chance of standing out from the crowd.

Best Value Fully Multi-Coated Bins
So to help with this and to help separate the wheat from the chaff, below I have listed a few of what I consider to be some of the best value for money fully-multi-coated binoculars around:

Low Cost
Mid Priced Mid-High High End
$50 – $130 /
£50 – £130
$130 – $300 /
£130 – £300
$300 – $500 /
£300 – £500
$500-$1200 /
£500-£1200

Pentax Papilio II 8.5x21

Hawke Frontier HD X 8x42

GPO Passion ED 8x42 Binoculars

Maven B1.2 10x42 Binoculars

More: View all Best Value Binoculars I have reviewed.

Additional Features of Lens Coatings

Beyond the core functionalities mentioned above, some manufacturers offer additional features through their lens coatings:

  • Scratch Resistance: Certain coatings can add a layer of protection against scratches and abrasions, extending the life of your binoculars.
  • Hydrophobic Coatings: These repel water droplets, preventing them from beading up on the lens and obstructing your view, particularly useful in wet conditions.
  • UV Protection: Coatings can be designed to block harmful ultraviolet (UV) rays, protecting your eyes during extended use.

A Note on “Ruby Coatings”
There are some binoculars that advertise that they have ruby or red multi-coatings. These are intended to reduce glare in bright light, or enhance specific colors in specific environments and has had some limited success in very specific applications.

But due to the fact that it “looked cool”, many low-end binocular producers started making ruby-coated binoculars to filter red to compensate for their poor-quality optics that do not properly converge the color spectrum. For more: Ruby Coated Binoculars.

Conclusion

Anti-reflection lens coatings on binoculars play a crucial role in enhancing your viewing experience and are important to take note of when comparing instruments. By understanding how they work and the features they offer, you can make a better decision when choosing the right binoculars for your needs.

Remember:

  • Look for binoculars with fully multi-coated lenses for optimal light transmission and image quality.
  • Consider additional features like scratch resistance, hydrophobic coatings, and UV protection depending on your intended use.

Further Reading:

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the very same for photography lenses and filters. Din’t know the exact meaning till now. Thanks. In spanish: “multi-capa”.

Most welcome – glad to help & thanks for the Spanish translation

[…] most of the models this includes the use of fully multi-coated optics with phase corrected BaK-4 […]

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