Binocular Lens and Prism Coatings

When out in the field with a group of fellow birdwatchers, amidst the peaceful sounds of sweet songbird calls and the excitement of spotting a rarely seen feathered friend, or even something as familiar as a chickadee or robin, the topic of binocular lens and prism coatings invariably comes up.

Technical terms are tossed around as we discuss both our current and someday future binoculars.

Binocular lens and prism coatings are key: Lenses should be fully multi-coated. Roof prisms should feature dielectric mirror and phase correction coatings.

Novice birders who attend these outings look utterly perplexed as we debate the optical superiority of specific coatings and applications and in the process, gently correct and learn from each other, expanding our knowledge.

I can clearly recall when, in my early days of birding, I also grappled with the basics, let alone the more complex elements. I wondered if my evolution of “successful birding” would require mastery of all things optics.

I had no idea that many aspects of optics, beyond the fundamentals, are often misunderstood by even the most experienced birders.

There are so many variables involved when discussing image quality that talking about coatings in isolation from other factors, like glass types, can lead you down a deep, dark rabbit hole.

In this article, I aim to demystify and educate by presenting basic optics concepts as they relate to optical coatings.

In addition, for those who are interested, I will carefully veer into some of the more esoteric aspects to give you a more complete understanding, and hopefully help you select binoculars that suit your needs.

Step by step we’ll discuss how different coatings used on lenses and prisms can increase light transmission, and cut down on chromatic aberration.

You will see that so much of it comes down to reflection, refraction, and the focusing of different wavelengths within the light spectrum.

Reflection And Light Loss

Let’s begin with a simple review of reflection and light loss, and their impact on light transmission and image brightness.

The two large barrels on the front part of the binoculars contain the Objective Lenses. Their job is to gather light and then pass it on as it passes through the various glass elements (the lenses and the prism) inside the binocular, ultimately providing you with a nice, bright image.

By following the red line in the Porro prism binocular illustration below, we can view the path of the light as it enters the objective lens at the bottom of the illustration and makes its way up to the ocular lens at the top of the illustration.

You might assume that the same amount of light that is gathered by the objective lens is the same amount of light that reaches your eye. And therefore you might also assume that all binoculars with the same size objective lenses will provide identical image brightness.

Similarly, perhaps you think that bigger objective lenses will always deliver a brighter image than smaller ones.

In fact, the amount of light that ends up reaching your eye is significantly impacted by a variety of variables, including those relevant to coatings, as well as glass types:

• the presence of prism and lens coatings
• the number of lenses that are coated
• the types of coating applied to those lenses, and the prism
• For lenses with multiple coatings, the number of coating layers that are used
• the type of glass the lenses and prism are made of

All of these factors can significantly impact light transmission. Without proper coatings and lens types, by the time the light reaches your eye, much of it would be lost.

Loss of light occurs at each air-to-glass surface point when light reflects off of each of the optical elements inside of the binocular. As light hits each uncoated piece of glass, up to 5% of the light is reflected, and so lost.

It might not sound like much, but when we consider the number of glass elements inside of a binocular, the total light loss can really add up!

Consider this: A typical pair of binoculars can contain 6-10 optical elements and up to 16 air-to-glass surfaces. If there were 10 sets of lenses, as much as 50% of the light (10x 5%) could be lost or “wasted” by the time it reached the ocular lens and your eyes.

In addition to light loss, all of those reflections increase the amount of light that is bounced around inside of the binocular, causing internal reflections. This dispersion of light results in images that are blurry, and wash out details by reducing contrast.

So, how do coatings reduce the amount of light that is lost? Let’s first start with a review of lens coatings, a bit of history, and how they work their magic.

Discovery Of The Impact Of Coating On Lens Reflection

In the 1940s, a group of German scientists from Zeiss discovered that by applying an extremely thin coat of magnesium fluoride to a lens surface, less light was lost as the light passed through the lens.

They found that the coating minimized light loss by reducing the reflective loss of the lens, which resulted in an increase in light transmission.

Therefore, coating the various lenses inside of a binocular (typically the objective and eyepiece lenses) will maximize the amount of light that passes through them, and ultimately into the user’s eyes.

Lens Coatings & Coating Layer Designations

The discovery the Zeiss scientists made was hugely important. However, in addition to the presence of coatings, the number of coating layers used will also significantly impact the amount of transmitted light. Why is this?

A single anti-reflective coating of magnesium fluoride, vacuum deposited on a lens, will cut down on reflection by increasing light transmission in one part of a light wave spectrum.

In contrast, multiple coatings on each lens surface can take lens light loss down even further by cutting down on reflection across a greater range within the spectrum. This is because each coating is composed of a specific material that is keyed to a specific color of light.

In fact, multiple coatings can reduce the light loss per lens surface to a tiny fraction – as little as .25% which, overall, can take full light loss down, within the binocular, to less than 5 percent. This means that at least 95% of light successfully makes its way to the eye.

In addition to less light loss, multiple coatings can also improve the optical image – details are more defined and colors are more true.

Coating Designations

There are various types of coating designations, based on the existence and number of coating layers that are applied to the lens. Again, this is a case where more is better. More layers of coating increase total light transmission, reduce reflections, and increase image contrast.

The more lenses that are coated, and the more layers of coating on those lenses, the better the image quality will be.

Proper Coating Application

There is a caveat when it comes to the effectiveness of binocular lens coatings – as with camera lens coatings when photographing birds, they must be properly applied to work their magic.

If the coating on a multi-coated optic is not applied evenly, or if the thickness of each layer of coating is off, the optical quality is significantly compromised.

Unfortunately, there is no manufacturing standard for full multi-coated optics. For example, a fully multi-coated binocular can consist of a few layers of coatings, all the way up to a total of 200 for the highest-end binoculars made by Zeiss, Swarovski, and Leica.

In addition, in certain markets, there is very little quality control of super low-priced binoculars that are marketed as fully multi-coated. This is one of many reasons to make sure that the binoculars you purchase are made by a highly reputable manufacturer.

An Easy Way To Check For Lens Coatings

There is no doubt that light reflectance is significantly impacted by coatings.

You can easily see if your binoculars contain lens coatings by doing the following

• Hold your binoculars in front of you, off to the left or right, with the objective lens facing forwards
• Make sure light is coming from behind you, towards the objective lenses
• Look at the reflection of light in the objectives
• Slowly move the binoculars
• Check the color of the reflection in the lens
  • Purple, green, or yellow reflections indicate the presence of coatings.
  • A mostly greenish color indicates a lens that is fully or multi-coated.
  • In contrast, a clear reflection means there are no coatings.

Additional Types Of Coatings

In addition to the types of coatings just described, binocular manufacturers may use other types of coatings as well,

• Water repellent coatings – Water repellant coatings are sometimes applied to the outside of the objective lenses and eyepieces. This is clearly a desirable option.
• Ruby coatings – Ruby coatings are to be avoided – they are used to eliminate red light from an image but unfortunately result in an image with a bluish-green tint. In addition, they reduce brightness and are highly reflective.

• Blue-removal coatings – These coatings are also to be avoided. They are used to remove the blue from an image and, in addition, to increase perceived image contrast. Unfortunately, they can also make an image look yellow.

• Specialty coatings – Some manufacturers have developed their own proprietary coatings for various optical devices. For example, Celestron’s proprietary StarBright XLT coatings increase transmission by up to 97.4%.

Now that we have finished our discussion about lens coatings, let’s talk about coatings for prisms.

Coatings For Prisms

Types of prisms

Specifically, there are two types of prisms: Porro and Roof. Since Porro prism binoculars often come with fold-down eyecups (which don’t allow for as much fine-tuning of eye relief as Roof prisms,) we will focus on Roof prisms in this section.

Types of Roof prisms

There are two types of roof prisms: Abbe-Koenig and Schmidt-Pechan. Since the majority of roof prism binoculars contain Schmidt-Pechan prisms, we will focus on these in our discussion of prism coatings (and, later on, prism glass types).

Mirror Coatings

Like the coatings for lenses, prism mirror coatings improve light transmission and so improve brightness as well as contrast.

In the Schmidt-Pechan roof prism, the mirror coating is particularly important. This is due to the way that light is reflected inside the lower prism during the second reflection.

When the light enters the lower prism, it undergoes two reflections. The first one is a total internal reflection. This means that no light is lost as the light bounces from one surface of the lower prism to another.

The second reflection then occurs on the bottom surface of the prism and feeds light into the upper prism. This second reflection occurs at what is referred to as “less than the critical angle”. This means that this surface does not have a total internal reflection. Without coating, the light would be lost during this reflection.

Therefore, mirror coating of the Schmidt-Pechan roof prism is deposited on the lower surface of the lower (half-pentaprism) prism in order to increase its reflectivity and so preserve light transmission.

Depending on the coating material used, preservation of light transmission can be astonishingly effective as the light passes through the prism

We can see that a dielectric coating retains almost 100% of the light, whereas aluminum and silver coatings retain less.

Phase Correction Coatings

Unlike Porro prisms, roof prisms suffer from an optical phenomenon known as “phase-shifting” – the peaks of the wavelengths passing through the prism shift out of phase from one another and so no longer line up.

The resulting image at the eyepiece is muddy with less detail and contrast, dimming of some colors and enhancement of others.

Technical descriptions of phase shifting can get quite complex. Unless you have a firm grasp of optics, discussions of concepts such as s- and p-polarization will probably evade and only confuse you further. Don’t worry – I won’t take you there.

Instead, before we jump into the wonders of coatings, let’s set the stage by beginning with a simplified overview of how light travels through a Schmidt-Pechan prism:

1. When the light beam from the objective lens hits and then enters the lower Schmidt prism component, it goes through its first internal reflection.
2. The reflected light travels to the bottom of the Schmidt (where the mirror coating should be) and goes through a second internal reflection.
3. The reflected light now travels out of the Schmidt, across a small air gap, and into the Pechan prism. The first internal reflection within the Pechan prism occurs.
4. The reflected light now travels up to the roof portion of the Pechan (where phase correction coating should be) where another internal reflection occurs.
5. The reflected light goes through one last internal reflection and exits out of the Pechan prism

Now that you understand the path of the light, as it travels through the two parts of the prism, we can go a little deeper.

One beam then travels faster than the other and, as it exits from the prism, hits the eyepiece lens a fraction of a second before the remaining beam.

When the two halves of light have both arrived at the eyepiece (again, one shortly before the other) and are recombined by your retina, interference between the light from the two paths occurs. This interference results in decreased resolution and contrast of the image.

The Application and Role of Phase Coating

So, in addition to mirror coating, just how does phase coating work its magic?

Multiple layers of a phase coating material are applied to the roof surface of the Pechan prism.

When the light beam reflects off of the roof surface and is split, the coating eliminates the phase shifting that would otherwise occur. Specifically, the coating corrects the out-of-phase light waves, allowing the peaks of the light waves to come back into phase.

As a result, when the separate beams then exit from the prism, travel through the ocular lens, and reach your eyes, the wavelengths are in phase and so there is no image distortion or degradation.

Manufacturers will indicate the presence and types of coatings for different binocular models, and you’ll find that most roof prisms binoculars have phase correction coating. Avoid roof prism binoculars that don’t!

Final Word on Binocular Lens and Prism Coatings

We have covered a lot of ground here, with several twists and turns. So many technical concepts – I know it can be hard to keep straight!

If we extract the bits that will guide you during the hunt for that perfect pair of binoculars, these are the main points to keep in mind:

• Look for binoculars with multi-coated, or, better yet, fully multi-coated lenses
• Stay away from binoculars with ruby- or blue-removal lens coatings
• Specialty coatings from a reputable manufacturer, designed to further increase light transmission, are also desirable
• Roof prism binoculars should have a phase-correction coating
• Roof prism binoculars should also have a mirror coating – those with dielectric coatings are best, followed by silver, and then aluminum coatings
• Water-repellent coatings will provide protection from the elements

And… last but certainly not least: I highly recommend purchasing binoculars made by a reputable company.

If you do, you won’t have to worry about the adherence to lens coating designations, the existence and type of prism coatings, and the quality of the glass that is used for lenses and prism glass.

You can order with confidence, and fully enjoy the excellent optical experience your binoculars will provide you with for years to come.

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