Many people focus on the magnification, objective lens size, or brand when selecting a pair of binoculars, a lesser-known but equally important aspect to consider is the type of prism glass used in their construction.
Binoculars are made up of several optical elements, including prisms, which are responsible for aligning the image and making it appear right-side-up. There are two main types of prisms used in binoculars: BK7 and BAK4.
In this article, we will discuss the differences between these two types of prism glass and their impact on the viewing experience.
BK7 Prism Glass
BK7 is a type of optical glass that is commonly used in binoculars. It is a borosilicate crown glass with an Abbé number of 64.2.
BK7 is a relatively hard material with extremely low bubble and inclusion content, while providing excellent transmittance throughout the visible and near-infrared spectra and down to 350 nm in the ultraviolet. It is a cost-effective option for binoculars and is commonly used in lower-priced models.
Properties and Performance of BK7 Prisms:
- BK7 prisms offer decent optical performance and image quality, making them suitable for general-purpose binoculars.
- They provide satisfactory light transmission, allowing users to view distant objects with clarity.
- Binoculars equipped with BK7 prisms are often more affordable compared to those featuring BAK4 prisms, making them accessible to a broader audience.
- However, BK7 prisms have limitations, particularly in challenging lighting conditions. They are more prone to internal reflections, which can result in a reduction of image contrast and overall brightness.
BAK4 Prism Glass
BAK4 is a type of optical glass that is considered superior to BK7. It is a Barium crown glass with an index of refraction of 1.5525 and a critical angle of 40.1°. BAK4 prisms are made of superior optical glass to produce clearer images.
They have a higher refractive index rate and a complete, circular shape of the exit pupil, which results in more vibrant and acute edges to your peripheral vision.
BAK4 prisms are considered the best type of Porro prism, as they are made of superior optical glass to produce clearer images. They are commonly used in high-end binoculars.
Properties and Performance of BAK4 Prisms:
- BAK4 prisms excel in light transmission, allowing more light to pass through the prisms and reach the eyes, resulting in brighter and clearer images.
- They provide exceptional image clarity and sharpness, making them ideal for activities that demand a high level of detail, such as birdwatching and astronomy.
- BAK4 prisms offer better light dispersion, leading to improved color fidelity and more vivid views of the observed subjects.
- Their most significant advantage lies in their ability to virtually eliminate internal reflections. This feature ensures superior image brightness, especially in low-light conditions, such as during dawn or dusk, or when stargazing under a moonless sky.
BK7 vs. BAK4: Which One’s Better?
The main difference between BK7 and BAK4 is the quality of the glass. BAK4 is considered superior to BK7 due to its higher refractive index rate and complete, circular shape of the exit pupil.
This results in better light transmission and edge-to-edge sharpness. BAK4 prisms are also more expensive than BK7 prisms, which is why they are commonly found in higher-end binoculars.
However, it is important to note that the choice of prism glass depends on the overall design of the binoculars. Shorter focal ratios or wide AFOV eyepieces demand a higher index of refraction, making BAK4 the better choice. On the other hand, BK7 may be the clearest, most defect-free optical glass available for prisms. It is also a cost-effective option for binoculars and is commonly used in lower-priced models.
Prism Glass Impact on Binoculars
The choice between BK7 and BAK4 prisms can significantly impact the overall performance and user experience of binoculars.
BK7 Prism Binoculars:
- Suitable for casual users, general observations, and outdoor activities.
- More budget-friendly, making them an excellent choice for entry-level users or those seeking affordable optics.
- Offers satisfactory performance in well-lit conditions, but may exhibit reduced image quality and brightness in challenging lighting situations.
BAK4 Prism Binoculars:
- Preferred by enthusiasts, professionals, and serious observers who prioritize optical excellence.
- Ideal for specialized activities, such as birdwatching, wildlife observation, and stargazing, where image brightness, clarity, and color accuracy are crucial.
- Premium optics with a higher price tag due to their superior optical properties.
Frequently Asked Questions (FAQs)
Understanding the differences between BK7 and BAK4 prism glass is important when choosing binoculars. BAK4 is considered superior to BK7 due to its higher refractive index rate and complete, circular shape of the exit pupil, resulting in better light transmission and edge-to-edge sharpness.
However, the choice of prism glass depends on the overall design of the binoculars and the intended use. BK7 is a cost-effective option for binoculars and is commonly used in lower-priced models, while BAK4 is commonly found in higher-end binoculars.
A Binoculars enthusiast, who love exploring skies and watching birds. It is my hobby to collect Binoculars of different kinds and try to explore the world through various lenses. This is all I do to explore happiness by magnifying my beautiful world.