What Is Bak4 Prism in Binoculars? – Here’s Everything You Need to Know About it!

People often ask what is Bak4 Prism in binoculars.

No need to worry because we are here to answer.

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What Is Bak4 Prism in Binoculars?

BAK-4 prism is the best kind of Porro prism. It produces clear images. BK-4 prisms produce acceptable images, but they’re not as good as BAK-4 prisms. Their images are more rounded than in BAK-4 prisms.

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Structure of Binoculars

Binoculars allow you to see far away objects without straining your neck. You can also use them to watch movies while sitting comfortably. Binoculars are made up of several parts including lenses, optical tubes, and eyepieces.


A prism is a device that changes the shape of light into different colors. Prisms are used in binoculars to correct the view horizontally or vertically.

Roof Prisms

Roof prisms have glass elements offset from each other. Roof prisms are used to correct images that are tilted or rotated.

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Porro Prisms

Porro prisms have glass elements in line with each other. Porro prisms can provide greater depth of focus and a wider field of vision than roof prisms. Porro prisms fold the light path, which makes them shorter and allows you to see further.


Technically, the type of prism used in binoculars is called a double-Porro prism but is usually referred to as just “Porro”.

It is also always capitalized because it is the last name of the inventor, Ignacio Porro, who designed the prism system around 1850.

The folded light path, which displaces the point where the light enter and exits the prism, results in the familiar look of traditional or old school binoculars.

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Other Prisms

The AK prism design is the most common but there are other designs that use different optical paths to achieve the same result.

The SP prism is smaller than the AK prism and therefore easier to handle. Zeiss uses SP prisms.

Pro tip: Porro prism binoculars are usually cheaper than roof prism ones.

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BAK4 Prism Binoculars

Porro Prism Binoculars can usually be distinguished by their offset tubes; the objective lens is not in line with the ocular lens. Roof prisms produce straight tubes, therefore these binoculars can fit into smaller cases than other types of binoculars.


  • Waterproof floating binoculars
  • Made out of stainless steel
  • Very durable and easy to use.

You can also buy these binoculars online.

Binoculars come in different shapes and sizes. There can be a huge range between seemingly similar pairs of binoculars. For example, B & H sells 10×42 binoculars ranging in price from $100-$500. The main reasons for this wide range are the quality of optics, coating applied to the lenses, other features that might be included, such as housing material. Additionally, prism type can also be a factor in determining price. Because of the physics involved in design and manufacturing the compact roof prisms, you can have a roof and Porro binoculars that seem identical as far quality and performance, but roof prism version will often cost more. The good news is that if form factor isn’t an issues, many people find that upgrading the quality of their binoculars by choosing a Porro prism without revaluating their budgets.

Things to Consider When Buying Binoculars


Binoculars are identified by their magnification and objective lens size. Magnification refers to how much closer things appear when viewed through binoculars. A 10x binoculars magnify objects ten times closer than what we see with our eyes.

Sports fans should choose something in the 3-5x range. Big game hunters should use something around 7x. Theater goers should get something in the 3-7x range.

Glass Prisms and Coatings

Binoculars made with eco-glass do not contain any toxic chemicals. You can be sure that when you throw away your binoculars, there won’t be any harmful waste left behind.

BAK4, or barium crown glass, is the best material for making prisms. It has a higher refractive index and lower refraction than other materials, which makes it transmit light better without losing too much light due to internal reflections

Price Range

Binoculars made out of BK7 are generally more expensive than those made out of BAK4. BK7 glasses have a square-shaped exit pupil, while BAK4 glasses have a rounded exit pupil. This results in better light transmission and sharper images.

SK15 glass is a type of material that strikes a middle ground between the previous two. It has a higher refractive index than both.

Minimum Focus Distance

Binoculars are used by people to observe objects that are far away. Birdwatchers use binoculars to view tiny details of birds.

Binoculars have a minimum focus distance of about 6 feet. As magnification is increased, the focus distance also increases. Users interested in a short close focus distance should look at larger objectives. Magnification should be kept at about 8X.

Focusing Types

The main focus ring is set on the bridge and moves the eyes symmetrically. Many manufacturers will have a diopter adjustment dial on one of eyepieces to adjust the focus to match individual prescriptions. Some models have the diopter correction integrated into the center focus mechanism.

There are three different focusing types

  1. Individual
  2. Focus-free
  3. Center-focused

Individual Focus

Individual focus models eliminate the center focusing mechanism to give each eyecup the ability to focus independently, but they are often frustrating to share and should only be considered for single users. Many marine and astronomical model features this system.


Focus-Free binoculars do not have any focusing mechanisms, relying on your eyes to focus on images. This allows you to concentrate on the view while enjoying the scenery.

Some users with exceptionally bad eyesight or weak eyes may want to avoid these models because they put a ton of stress on the eyes and can cause discomforts such as eye strain or headaches.


Center-focused binoculars produce sharp images. You can move lenses simultaneously and adjust the focus.

what is bak4 prism in binoculars

What Is bak4 Prism in Binoculars?

Weather Resistant Binoculars

Weatherproof binoculars are designed to withstand harsh weather conditions. Some binoculars are rated as waterproof and fog proof. These types of binoculars must be protected from moisture.

Water getting into the optical tubes can cause condensation on the inside of the lenses, which interferes with vision. Eventually, this can lead to internal rust and corrosion within the binoculars.

Chassis Materials

Chassis is the frame of the optical device around which the whole optic system is built. Aluminum is mostly used on the market today. Aluminum is lightweight and strong, inexpensive, easy to work with, naturally corrosion-resistant, and a bonus is that it is also naturally transparent.

Specialty Binoculars

Binoculars are tools that allow people to see things far away. They use lasers to measure distances. Some binoculars can make measurements in feet or meters. They can also show the distance to a target in real-time.

Final Thoughts 

Binoculars are used for many different things. From watching birds to looking at stars, there is an instrument for everyone. There are also many types of binoculars available. Some are better than others depending on the type of use. A telescope or spotting scope may be more suitable for observing objects close up. For general observation, a pair of binoculars is perfect.

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  • John Moses

    John is the Editor in Chief here at The Outdoor Stores. His area of expertise ensures that there is no one better to suggest which rifles are most suitable for your hunting experience. He is also available for you to contact him personally to discuss the types of animals you want to hunt and the terrain you will be hunting on. Feel free to read his posts for expert opinion on Rifles, Scopes, Rangefinders, Bonoculars and Monoculars.

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