Binoculars are essential for many activities, such as hunting, birdwatching, and stargazing. The prism system, one of the most important components of binoculars, corrects the image’s alignment to prevent it from being upside-down. BaK-4 and BK-7 are the two varieties of prism glass that are discussed the most. However, what distinguishes them, and which one best suits your requirements? Let’s look at the details.
BaK-4 Prism: What is it?
Barium crown glass, or BaK-4, is valued for its exceptional optical qualities, which make this kind of glass well-known. BAK-4 has a refractive index of roughly 1.568, which is regarded as high. This guarantees optimal light transmission and lessens interior light scattering. It also has a homogeneous structure, that removes the dark edges frequently observed with BK-7 glass and provides edge-to-edge sharpness. Thanks to its structure, BaK-4 glass improves clarity and reduces aberrations.
Excellent contrast and bright, clear images make it perfect for astronomy, hunting, and birdwatching when image quality is crucial.
BaK-4 Glass in Porro and Roof Prisms
As you may have learned, binoculars use prisms in two configurations: Porro and Roof. BaK-4 enhances both designs.
BaK-4 glass has a high light transmission percentage and low optical distortions, but its effectiveness depends on the prism type.
The light path is shorter and there are fewer optical losses in Porro prisms. This enables BaK-4 glass to produce images with colorful edges and sharp, bright images, showcasing its full potential.
The design is more intricate on Roof prisms. Higher reflection densities and more coatings lengthen the light path, increasing light losses. Roof prisms made of BaK-4 glass nevertheless produce vivid images, although these light losses cannot be entirely compensated for.
Because of their better construction, BaK-4 glass therefore exhibits its full potential in Porro prism systems. Its benefits are still noticeable in roof prisms, particularly when combined with superior coatings.
What Is BK-7 (BaK 7) Prism?
The term “BK-7” describes borosilicate glass, which is frequently found in inexpensive binocular prisms. Among its main attributes are:
- Refractive Index: Usually less than BaK-4, at 1.516. As a result, internal reflection is less effective, and light transmission is somewhat decreased.
- Physical Characteristics: It is inexpensive and lightweight, making it appropriate for a variety of activities.
- Optical Impact: A lower refractive index may result in slightly less clear images and vignetting, which darkens the edges of the field of view.
Benefits and Drawbacks of BK-7
The primary advantage of binoculars with BK-7 glass prisms is their affordability. If you plan to use binoculars infrequently and have modest demands for image quality, they can save you money. At the same time, BK-7 glass allows for the design of lightweight and fairly compact binoculars with acceptable image quality. At least it’s optical glass, not plastic. You didn’t know that binoculars have plastic optics either?
However, prisms made of BK-7 glass have some downsides. Reduced brightness and clarity of the image are the most noticeable. In bright daylight, this might not be apparent, but in twilight, you’ll certainly notice the difference. For birdwatching enthusiasts, I recommend only binoculars with BaK-4 prisms. The lack of brightness also complicates astronomical observations.
What is ED Glass? Is it better or worse than BaK-4?
ED glass (Extra-Low Dispersion) is premium optics, which means that binoculars with ED glass are expensive. It is designed to eliminate light dispersion, which leads to chromatic aberrations and color distortions.
Light dispersion is the decomposition of light into spectral colors when passing through an optically dense substance due to the dependence of the refractive index and the speed of light in the substance on the frequency (or length) of the light wave
Dispersion is especially strong in binoculars with a long focal length – such as those used in astronomy. Thanks to the usage of ED glass, the image does not suffer from chromatic aberration (color fringing) and light distortions. However, this glass is not used in prisms, but in binocular lenses. So they have different purposes.
ED lenses, BaK-4 prisms, and optical coatings work together to produce a remarkable optical system that combines excellent color fidelity and clarity.
BaK-4 Relative to K9 Glass
There is another type of optical glass, although less popular: K9 glass, also known as optical borosilicate glass. It is used for both lenses and prisms and is commonly found in budget optical devices, including binoculars, telescopes, and laser systems. When used in prisms, K9 offers sufficient transparency and low defect levels for basic optics. However, compared to BaK-4, K9 has a lower refractive index, which reduces brightness and image sharpness, especially at the edges of the field of view.
Refractive Index. Like K-7, K9 optical glass has a refractive index of about 1.515 (compared to 1.568 for BaK-4). The amount of image distortion is higher.
Optical Transparency. K9 provides moderate transparency, comparable to BK-7, but inferior to BaK-4.
Physical Characteristics. K9 is slightly heavier than BK-7 glass, which increases the weight of optical devices.
K9 and BK-7 are suitable for entry-level, but provide a dimmer image with more distortion to be used in professional binoculars.
And little more on Binoculars with BaK-4 Prisms
As we have discussed, K9 and BK-7 optical glasses are suitable for beginners or short observations in clear weather. However, they are not the ideal option for use at low light levels due to their excessively low light transmission. In addition, binoculars with BAK-4 prisms can be used for a variety of purposes. BaK-4-equipped small versions offer crisp, clean photos for birding. They are especially useful for hunting since they ensure sharp photos even in low-light situations in the forest. High-aperture objectives and BAK-4 glass enable binoculars to study planets and efficiently catch light from near-space objects in astronomy. Even at 8x magnification, the moon image is breathtaking! Not to mention the use of professional astronomical binoculars with 15-20x magnification.
Although a special anti-reflective coating is used to increase brightness, it is not recommended to use BK-7 or K-9 prisms for twilight observation: the image will be too dark and you may not see any details at all.
We will talk about coatings of optical glass in other article.
Comparative Table: BaK-4 vs BK-7 vs K9
| Feature | BaK-4 | BK-7 | K9 |
|---|---|---|---|
| Clarity | Excellent | Good | Moderate |
| Brightness | High | Moderate | Moderate |
| Cost | Higher | Affordable | Low |
| Applications | Professional | Casual | Entry-Level |
Conclusion
From all of the above, we can draw the following conclusion: buy binoculars with BK-7 glass prisms if you do not know what you need binoculars for. There is no point in spending money on technologies whose effect you may not even notice.
For an inexperienced user, if you observe during the day and at a short distance, the difference may not be noticeable at all. But if you plan to use binoculars in low-light conditions, if you want to not only see the object itself, but also appreciate the beauty of nature, or if you want to do astronomy, then BAK-4 prisms are your choice.
I didn’t know that they use plastic in optic omg