Binocular Lens And Prism Glass

Many novice birders, looking to purchase their first pair of binoculars, often make a decision based on price alone. Else, they are understandably so overwhelmed by the number of ever-expanding features to consider, they don’t have the patience or confidence to educate themselves.

In addition, if they simply research the best binocular recommendations for birding, they will see a huge number of recommendations. However, they may not realize that it’s important to select binoculars specific to their individual needs.

For example, if they wear glasses, and plan to birdwatch in daylight hours, they might want to limit their search to the best 8×42 binoculars for people who wear glasses.

In addition to filtering the binocular search according to specific needs, optical excellence is definitely an important consideration (though I think you can break this rule when purchasing binoculars for kids who have recently discovered the excitement of birdwatching).

It turns out that the type of binocular lens and prism glass that is used significantly impacts the quality of the image.

The good news is that you absolutely don’t need to be an optics expert to know what you should look for when making that first purchase, or even when upgrading to a higher quality pair of binoculars.

In general, lens should be made of a low dispersion glass, such as ED/HD, Fluorite, or APO. Binoculars should use BK7 or Bak4 prism glass, or SK15 glass. But it’s worth knowing the optics behind all of this, as well as why you should buy from a reputable manufacturer.

Read on to learn everything you need to know about binocular lens and prism glass, so you can be an educated consumer.

How Many Optical Elements are Used in Binoculars?

One of the reasons that glass quality is so important in binoculars is because binoculars are chock full of lens and prism glass.

The glass elements inside of the binocular typically consist of the objective lens, the focusing lens, the 2 prisms, and the ocular lens. So, the typical binocular consists of 3 lenses, and 2 prisms.

However, when counting the actual optical elements, it becomes a bit trickier since lenses may consist of groupings of one or more lens elements.

In fact, a binocular with the maximum amount of optical glass, taking both barrels into account, can consist of over 20 optical elements.

LENSES and PRISMS	NUMBER OF GLASS ELEMENTS
Objective Lens	2-4
Focusing Lens	1
Field Flattener Lens	2
Ocular Lens	3-7
Prism	2

Objective Lens (2-4 elements)

The job of objective lenses is to capture light, and then flip and invert the image. When the objective lens diameter is large, more light will be capture. Objective lens configurations are typically of two types: an achromatic doublet or an apochromatic triplet.

Achromatic Doublet (2 elements)
A typical lens configuration for an objective lens of a Roof prism binocular is what is referred to an achromatic doublet. The doublet often consists of 2 glass lenses cemented together.

One of the lenses is convex and made of low refractive crown material, while the other is concave and made of high refractive flint.

Apochromatic Lens Triplet (3-4 elements)
Objective lenses may even be comprised of an apochromatic lens triplet, comprised of 3 lenses – often a cemented doublet and a singlet (also called a 1+2 configuration), and much more rarely, a combination of 4 lenses.

Focusing Lens (1 element)

The focusing lens allows the user to focus on an image. As the user turns the focus wheel to the left and to the right, the lens moves back and forth.

Field Flattener Lens (2 elements)

Binoculars may also feature a 2-element field flattener lens. The job of the field flattener lens is to improve image quality by enhancing sharpness and decreasing image distortion.

Ocular Lens (3-7 elements)

The job of the ocular lenses is to magnify the image that has traveled through the objectives and prisms.

The ocular lens (also called the eyepiece) may be comprised of 3-7 elements. A typical configuration is a singlet + doublet + singlet (also called a 1+2+1).

Prisms (2 elements)

The job of the prism is to shorten the optical path and turn the image from the objective lens right-side-up and flip it from left to right. Both Roof and Porro prisms consist of 2 aligned prisms.

The Impact Of Binocular Lens and Prism Glass

Let’s begin by talking about the impact of binocular lens and prism glass.

In addition to lens coatings, the type of glass that the lenses are made of can significantly increase optical excellence by decreasing reflection and so increase light transmission.

In addition, the quality of prism glass also impacts image quality.

High-quality glass, such as BaK 4 prism glass, increases light transmission by cutting down on internal reflections as light bounces around in the prism. This results in brighter images as more light travels through the prism and on to the ocular lens.

High quality prism glass also decreases chromatic aberration because it cuts down on light dispersion. This means that different wavelengths (colors) will have less separation during refraction.

Let’s begin with a discussion of chromatic aberration so our subsequent review of optical glass will be more meaningful.

Chromatic Aberration in Lenses

What do we mean by chromatic aberration and why is it not a good thing?

Specifically, as light passes through lens glass, the different colors (or wavelengths) of the white light are bent by different amounts. This is known as refraction (different from reflection!) and is similar to the bending of light you see when light passes through a prism.

Now that we know what chromatic aberration is, let’s get back to our discussion of different low-dispersion glass types for lenses!

Impact of Lens Glass Types and Designs

ED Glass

ED (extra-low dispersion) glass (also referred to a high density – HD – glass) is a high-quality optical glass specifically designed for high light transmission and low light dispersion. It is composed of a specific formulation that contains rare-earth elements.

ED glass lenses, as well as multiple coating layers, cut down on chromatic aberration or “color fringing”, which is due to the splitting of the light spectrum.

Objective lenses made with ED glass are designed to deliver nice bright, razor-sharp images, true-to-life vivid color, and help to focus the light waves of the color spectrum on your eyes.

As you can imagine, this makes the viewing experience more pleasurable, as well as assisting with bird identification.

ED Glass and Lens Quality

Unfortunately, not all ED glass is created equal. Since there is no ED standard that specifies the components that must be used in the ED manufacturing process, the quality of ED glass can vary.

Since there is no standard that must be adhered to, less reputable manufacturers will cut back on the number of materials they use. As a result, images are not as sharp, and colors are not as vivid.

Lens designs That may Use ED Glass – Achromatic and Apochromatic

Achromatic Lenses

An achromatic lens is comprised of 2 glass lenses cemented together (and so referred to as a doublet lens).

Each of the 2 glass lenses (referred to as glass elements) of the achromatic lens work on a different color wavelength (typically red or blue) of the light spectrum, correcting for chromatic aberration.

As the 2 different colored lights of the spectrum refract (or bend) at different angles, the different shapes and materials of the achromatic lens elements help to bring them into focus at the same point.

Interestingly, ED glass is usually used for one of the 2 glass elements that make up an achromatic lens.

Apochromatic Lenses

Apochromatic (APO) glass corrects for chromatic aberration at 3 wavelengths and so has greater correction than the more common achromatic glass.

The Apochromatic is typically comprised of 3 glass lenses that are cemented together (and so referred to as a triplet lens). Each of the 3 lenses (referred to as glass elements) work on the blue, red, or green wavelength of the light spectrum.

As the different colored lights refract (or bend) at different angles, the APO glass helps to bring the lights into focus at the same point.

Interestingly, among the 3 lenses used in the APO triplet, at least one is usually ED glass.

Apochromatic glass is more expensive and less common than achromatic glass.

Now that we’ve reviewed the types of glass used for lenses, let’s discuss the types of glass used for prisms.

The Impact Of Prism Glass Types

Prisms are also composed of high-quality optical glass. The first two types presented below – BK7 and Bak4 prism glass – are glass designations used by Schott AG, a highly respected German optics authority. The last type – SK15 – is used less often than BK7 and Bak 4 prism glass types.

These glasses different in their refractive indexes and Abbe numbers. The higher the refractive index, the higher the light transmission and brighter the image. The higher the Abbe number, the less color dispersion and chromatic aberration.

Importance of Exit Pupil for Determining Prism Glass Type

The shape of something called the exit pupil can help you determine if your binoculars use BK7 or BaK4 prism glass. But before we dive into the method for determining prism glass type, lets first define what we mean by exit pupil and how it is measured.

The exit pupil is the width of the beam of light that leaves the ocular eyepiece (the eyepiece closest to your eyes). When the objective lens diameters (the large lenses at the front of the binoculars) are large, they will produce a wider beam of light (exit pupil) that leaves the eyepiece.

The width, or diameter, of the exit pupil typically ranges from a tiny 1mm light beam to a much larger beam of 20mm. Why do we care about the size of the exit pupil? The beam of light that exits the ocular lens will make contact with the pupil of your eye and so determine the perceived brightness of the image.

During the day, the pupil of the eye opens up to 3mm. If the exit pupil is 3mm or more, the image perceived by the user will be nice and bright.

In addition, binoculars with exit pupils that are larger than 3mm will be more comfortable to use since the placement of the binoculars, and so alignment of the exit pupil with the eye pupil, can be less precise. There is more wiggle room.

But what happens in low light conditions? This is where it gets interesting – During dawn or dusk, the diameter of the exit pupil becomes a bit more critical. This is because the human eye will dilate to an even greater extent in low light conditions in order to capture more light, and dilation of the eye is age dependent.

The larger the dilation of the eye pupil, the more use the eye can make of the light delivered by a larger exit pupil.

Here is where age comes in: A child’s eye pupils can dilate to a maximum of 7mm. In this case, you want the diameter of the exit pupil to be at least 7mm for a nice bright image and absence of light loss. No problem.

However, interestingly, as adults age, maximum dilation of the eye pupil can decrease to 4mm. Therefore, compared to the 7mm dilation of a child’s eye pupil, there will be less light that is able to be utilized by an adult’s much smaller eye pupil.

This means that, in lower light conditions, much of the light produced by a 7mm exit pupil will be lost when it hits a human adult eye dilated at 4mm. The 4mm eye pupil is simply too small to make use of all of the light in the 7mm exit pupil.

Therefore, in low light conditions, the image the child sees will be brighter than the image seen by the adult since the child’s eye can more fully benefit from the larger beam.

How to calculate the diameter of a binocular’s exit pupil

To calculate the diameter of the exit pupil, divide the diameter of the objective lens by the magnification power. So, for example, for an 8 x 42 binocular, you would divide 42/8 which equals a 4.2mm exit pupil.

Now we can dive into the method for determining binocular prism glass.

Using exit pupil shape to determine binocular prism glass type

1. Turn your binoculars around so the objective lenses are facing you
2. Hold the binoculars about 8” away from your eyes
3. Look into the objective lenses – you should see a white dot. This is the Exit Pupil
4. If the Exit Pupil looks nearly circular, your binoculars have Bak4 prism glass
5. If the Exit Pupil is looks more squared off, your binoculars have BK7 prism glass

BK7 Prism Glass

BK7 is a type of boro-silicate crown glass that is highly transmissive, and easy to manufacture. It’s the most widely used prism material for binoculars.

The main difference between BK7 and BaK 4 prism glass, is that BK7 glass is made of lower density optical glass, and therefore have a slightly lower refractive index. Lenses with lower refractive indexes bend light less efficiently and so are not as effective as preventing light loss.

When compared to BaK4 prism glass, BK7 images also have a bit less contrast, and the color fidelity is not as true. However, they still have excellent light transmission properties. BK7 glass is found in mostly lower priced binoculars.

BaK4 Prism Glass

BAK-4 is a light barium crown glass (BAK stands for BaritleichKron, which is German for “Barium Crown”).

BaK4 prism glass is considered to be the best type of prism material. They are comprised of a higher density optical glass than BK7 glass and so are more light transmissive. They transmit light better without light being lost due to internal reflection

When compared to BK7 glass, the combination of the higher refractive index (which bends light more efficiently,) relative lack of imperfections, and circular shaped exit pupil of the Bak4 prism glass results in even clearer and brighter images, and truer color fidelity.

They are also more effective in eliminating dispersion.

BAK-4’s are more challenging, and therefore more expensive, to manufacture, but image quality is exceptional. They are mostly found in medium- and high-priced binoculars.

All BaK4 Prism Glass is Not Created Equal

The following factors are critical enough so that poorly manufactured BAK-4’s are less desirable than BK-7 prisms of high quality.

• While Schott Bak4 prism glass is made with Barium Crown glass, cheaper Chinese-made Bak4 glass is made with Phosphate Crown that also has a lower refractive index and dispersion (though higher than BK7 glass)

• High quality BaK4 glass should be correctly sized, properly polished with blackened sides and grooved hypotenuses, and should feature shielded reflective surfaces. In addition, it should be secured into the binocular housing with a prism cage.

All things being equal, unless you are using binoculars with a wide field of view and small exit pupils (where BK-7 will not be optimal), you actually may not notice a significant difference between BK-7 and BaK4 prism glass.

Here is an example of an image viewed through both glass types where the difference is quite noticeable:

SK15 Prism Glass

SK 15 glass is less commonly used than other glass types.

It has a higher refractive index (transmitting more light) than BK7 and Bak 4 prism glass, but its dispersion measure falls between these two glass types.

A high dispersion number means that when the light passes through the optical glass, there will be less separation between the wavelengths, and so less color dispersion and chromatic aberration.

SK15’s produce an image that is very clear, with high contrast.

Final Word On Binocular Lens and Prism Glass

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 about binocular lens and prism glass that will guide you during the hunt for that perfect pair of binoculars, these are the main points to keep in mind:

• Lens should be made of a low dispersion glass, such as ED/HD, Fluorite, or APO
• Prisms should be made of BK7, or BaK4 prism glass, as well as SK15 glass

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.

If you would like to learn more about the many variables to consider during your binocular purchasing research, you can read more here.

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