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Binoculars Buying Guide

Binocular and its Features

What is a binocular?

Binoculars are nothing but a handheld device which comprises of a series of prism and lenses that allows the viewer to magnify the subject and the field of vision. This helps in better viewing of things which are far off or distant and can be visualized by using both eyes.

The various features found in a Binocular are the numbers that are associated with them, the field of view, Exit Pupil, Relative Brightness and Twilight Factor, Prisms, Lens Coating, Focusing, Diopter Adjustment, Interpupillary Distance, Close Focus, Eye Relief, Weight, Waterproof, Armoring, Tripod Mounting and the Application.

Check list before buying a binocular

Binocular Numbers

All the Binoculars are usually marked with 2 numbers and some may have 3 numbers and they are called as zoom binoculars. The first number normally denotes the magnification level of the Binocular. In a binocular which is numbered as 8×40, the 8 shows that the magnification of the image is about 8 times closer.

In a zoom binocular where there are three numbers like 7-15×40, the first two numbers denote that there is an option for fine-tuning the binocular’s magnification from 7 times to 15 times closer.

The magnification of a binocular required to choose carefully for a hand-held binocular, because when the magnification increases not only the image size becomes closer but also the finest details and the shaking and trembling of the hand is been taken into account. Naturally, the image brightness goes down along with the steadiness. A maximum of 10x magnification can be used for a handheld binocularly. More than that definitely requires a tripod for support.

The second number is the diameter of both the front lens which is in millimeters. This is the one which determines the performance. The difference between both 8×25 and 8×40 binoculars is that the more the diameter is, it enhances brightness and sharpness of the image. Wide beams of light are left from the eyepieces which make viewing comfortable with a large diameter.

On the other hand, if the diameter is more then the lenses are heavy and finally make the binocular also heavy to carry. Devices with smaller diameters make them light weighted and compact enough to carry while on travel.

This second number of the binocular definitely does not indicate the field of view which is a mistaken and common opinion of many.

Field of View

In a binocular, field of view is nothing but the area seen through the lens. It is determined by the magnification and the eyepiece. The larger the magnification is then the more detailed view is perceived but less area. A 10x show more clarity than an 8x while the latter allows more area to be visualized. So it all depends on the need to view the user wants. The eyepiece is yet another factor which decides on the field of view. Wide-angled eyepieces though expensive are of excellent optical quality that gives you sharp images.

Exit Pupil

The binoculars have a beam of light that is erupted from the eyepiece which is measured in millimeters and the actual width of it is known as the exit pupil. When held at an arm’s length, a circle of light can be seen in the center of each eyepiece. This is calculated as the exit pupil. The exact size is calculated by dividing the first number of the binocular with the second number which gives you the size of the exit pupil. For example, 7×35, 10×50 and 8×40 all have the same exit pupil size of 5 to it.

The exit pupil shows brighter images in low light also if it has large exit pupils. Normal viewing requires only 2.5 – 3 mm exit pupil. Nevertheless, astronomical requirements need 5-7 mm only. More than 7 mm of the exit pupil is not preferred since the human eye cannot extend or accept such a large exit pupil. But the traditional marine binocular uses a 7×50 which has an exit pupil of 7+ which can be kept centered on the eye as on the deck of a moving ship or a boat.

RE or Relative Brightness

Relative brightness is also an amateur guide to the brightness of the images like that of an exit pupil. The RE is calculated by squaring the exit pupil of the binocular. If the exit pupil is 5 mm then the RE is 25.

Twilight Factor

The twilight factor of a binocular is determined by a simple mathematical formula that calculates or predicts the amount of clarity or detail that can be visualized in very dim or low light. It is arrived by deriving the square root of the magnification factor with that of the objective lens. The square root of 10×40 is 20 (20×20 = 400) and so the twilight factor is 20. This is also only a rough estimate and should not be taken too literally to evaluate the quality of the binocular.


The prism of a binocular is the glass piece or element that is used to get an upright and perfect image corrected right to left. Usually, binoculars use either a Porro prism or a roof prism.

A Porro prism has the front lens and the eyepiece always set apart and never in a straight line. Optically speaking this is more efficient and less expensive than other types. A Porro prism is quite larger and bulkier and it can be easily identified by its size and design. This is an older style and version of the binoculars.

The roof prism is much smaller and it has a shape like that of a small house with a roof on top. This is much smaller and compact in to handle than that of a Porro prism. It feels good to hold and handle a roof prism binocular though it is expensive. A roof prism binocular can be easily identified as both the eyepiece and the front lens is either aligned in a straight line or a straight tube.

Optically, the binoculars which possess sharp roof prism are phase-corrected and the bright roof prisms do have silver coated mirrors to yield better results. The precise alignment of the roof prism benefits as it is kept safely in a cage like and the focusing mechanism is found to be on the inside of it. This helps in more rugged and smoother focusing in a roof prism binocular.

The different grades of the optical glass that are used as prisms are either Bak4 or BK7. Bak4 is usually used only by high-quality binoculars and the BK7 is considered to be of inferior quality used in cheap or inexpensive binoculars.

Lens Coating

The chemical coating that is found on the lenses of a binocular which produces bright images known to be lens coating. This coating is required as whenever a beam of light strikes the lens a percentage of it is lost. To avoid loss of light the lenses are given a chemical coating.

The three different types of coatings are the fully coated or the multi-coated or the fully multi-coated.

When a single layer of magnesium fluoride is coated over the lenses it is called as a fully coated binocular. This is the oldest method used and is also found to be the least efficient. The images produced by this binocular are not very bright and it is usually used only in cheap binoculars.

When multiple levels of coating with special chemicals are given to the lenses they allow maximum light to pass through them. This is best in most of the handheld binoculars and they are easily known as multi-coated binoculars. They are certainly brighter than normally fully coated ones.

When a binocular has all its lenses covered with multi-coated then it is known as FMC or fully multi-coated binoculars. This allows the brightest possible image and always used in all the quality binoculars manufactured.


The focusing of a binocular has three types to it. The binoculars with center focus have a single wheel which helps to focus on the subject. This allows focusing on both far off and nearby objects and is one of the versatile focusing systems for a binocular.

The IF or the Individual Focusing binoculars have separate focusing for both the eyepiece. This works well with the medium and long-range images but does not suit for close vision. These are mainly used for the marine binoculars and the astronomical binoculars.

The third one is a cheap version of the individual eyepiece focusing binocular. In these, both the eyepieces are already present and no further adjustments could be done individually to them. These binoculars cannot be focused any more than forty yards and this happens to be a handicap since most of the people have one eye a bit stronger than the other. This type of binocular does not correlate both the eyes vision individually. They are otherwise called as “no focus” or “focus free binoculars”.

Diopter Adjustment

This is a feature which allows adjusting the difference of vision between the right and left the eye as always one is greater than the other. The diopter adjustment is usually found in all the binoculars other than the focus-free ones. Traditionally the adjustment was placed in the right eyepiece but in the latest models, it is a separate wheel or a locking mechanism that is located on the center focus knob.

Interpupillary distance

An Interpupillary Distance or the IP of a binocular is the exact length or distance from the pupil of the eyes. Most of the binoculars have the option to make adjustment accordingly to the width of the face but sometimes persons who have smaller faces or ones who have larger faces may find it difficult to get them fitted into the binoculars properly.

Close Focus

In a binocular, there is a possible distance the device will be able to focus. The maximum closeness that is accommodated by the binocular is known to be the minimum or close focus of a binocular. This is mainly used in bird watching.

Eye Relief

The Eye Relief of a binocular is the most possible distance that a person can place their eyes from the eyepieces and still be able to view clearly. This mainly noted for those who wear spectacles. Their glasses would prevent them to get close to the eyepieces. They have to be careful and if they wear a thin plastic lens they would be able to maintain about 13 mm distance. But if they wear thick and heavy glasses, it would be preferable to go for a binocular which has an eye relief of up to 17-20 mm range.

It is advisable to push the eyecups to a down posture when the person wears spectacles and revert back to up when not wearing them. In olden days there was a rubber fold down as eyecups but the modern and latest binoculars have either a twist up or pull up styled eyecups.


Usually, a binocular of 30 – 35 ounces is normal to carry around without any discomfort. But if pain develops over the neck, it is advisable to loop the binocular with a shoulder harness which shifts the weight to the shoulders from that of the neck.


When a binocular is said to be waterproof, it is then fully filled with nitrogen on the inside of it. A waterproof binocular is always fog-proof inside as moisture can splash on the outer surface but it should not get collected inside. Waterproof is not exactly water-resistant or climate-proof or rain-proof. If the usage is preferred for harsh conditions it is better to go for a waterproof one as it is tightly sealed and does not allow dirt or dust collection also into it.


When a binocular is covered or housed within a rubber or any other synthetic covering which protects it from scratches or from accidental bumps and bangs, then it can be said as an armored binocular. This should not be mistaken for waterproofing. This just makes it quite comfortable and safe for the binocular.


A binocular is mounted over a tripod if it comes with a tag which says it to be tripod adaptable and if there it is threaded for the use of a tripod adapter. Sometimes the adapter had to be purchased separately. The tripod socket is normally found on the front of the center hinge, where the binoculars normally fold. It is kept hidden under a cap. Binoculars with larger magnification normally need a tripod to steady them for shaking by holding them in hands. Even heavy binoculars with a heavy objective lens of more than 70 mm would need a tripod in order to balance the added weight.

To conclude, the binoculars have a long history and have been a good companion to a man in his various pursuits of nature via invention and discovery. Binocular lovers or owners might be still curious to know the various details about them and would be keen on trying to understand their device in a much better way.

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