Understanding Binoculars Specifications

Binocular simply means a pair of device that magnifies objects. That’s the primary purpose of binoculars and they comprise two of telescopes mounted side by side with both focused at one point. inside each telescope is an objective lens, prisms and eyepiece lens. These parts function together to create beautiful images of the objects you are viewing.

Every binoculars are described using a pair of numbers and other specifications. Understanding binoculars specifications is important in determining the binocular performance and in selecting the right kind of binoculars for your needs.

Understanding Binoculars Specifications

Below are the binoculars specs that you often find on any binoculars description page.

Binocular Magnification

On every pair of binoculars, there is a pair of numbers separated by a multiplication sign. These numbers represent the magnification power and the objective lens diameter size respectively. For example ‘8×42 mm’. The 8x is the magnification of that particular binocular.

This magnifying power is indicated to you how many times the object you are viewing is magnified. This simply means the object will appear eight times closer to you than your apparent eyes.

There is a misconception that high magnification means better. don’t be deceived it doesn’t necessary means so. in fact anything more than 12x going will require you to use a tripod to get a steady view. Because all your body movements are exponentially magnified as well. High magnifying power also gives you narrow field of view as well.

Understanding Binoculars Specifications

This is said with the exception of image stabilized binoculars. For those use special technologies to stabilize high powered binoculars.

Binoculars with magnification from anything to twelve times is easier to handle with hands and move around but still get clear images.

Objective Lens Size or Aperture

The objective lens size is also known as the aperture which represents the diameter of each of the objective lenses. This number affects the size of the objective lens and its light gathering power. Since the function of the objective lens is to gather lights from the object.

This number is usually printed with the magnification on the binoculars. It is always the second number with the magnification been the first. let’s use the example I used above; ‘8×42 mm’. The aperture is the 42mm. It is the diameter of the objective lens and its always indicated in millimeters.

This binocular specification determines the resolution and brightness of the image. However, the higher the objective lens size, the heavier and bulkier your binoculars will be. As it always turns out that the objective lens size affects the size of the binoculars drastically.

Binoculars Eye Relief

Eye relief is the optimal distance from the eyepiece to your eye or the focal point where the light passes through the ocular lens (eyepiece). It is the comfortable distance between your eyes and the binocular eyepiece such that you can see a clear image through your binoculars.

It is important to have a larger binocular eye relief if not, you will get eyes fatigue if you are using them for a longer period. These days, most manufacturers place eyepiece caps on the eyepieces to create enough room for you to see comfortably.

For those who are glass wearers, manufacturers are taking into consideration your special needs by offering dioptric adjustments on the eyepieces that will allow users to fine-tune the focusing system to their eyes prescriptions so they can use the binoculars without glasses.

However, if the binoculars do not have a dioptric adjustment, most manufacturers ensure that there is enough eye relief for you to be able to wear your glasses and use your binoculars. The eye relief affects the field of view. The larger the eye relief is, the narrow your field of view will be. with a high level of optical engineering,  there are able to get both large eye relief and wide field of view but at an expensive price.

Binoculars Exit pupil

The binoculars exit pupil is the objective diameter divided by the magnifying power. This number indicates how bright an object will appear when viewed in low light situations.  A higher number means brighter images. A large exit pupil also makes it easier to maintain a full image of an object if your hands move or shake. Exit pupil size is measured in millimeters.

For standard daylight viewing, exit pupil size is less important. In bright light human pupils narrow to roughly 2mm.

Example: For 7 x 35 binoculars, 35 divided by 7 equals an exit pupil diameter of 5mm.

In dim light, however, our pupils can widen up to 7mm. A 7 x 50 binocular, for instance, offers an exit pupil size of 7.1mm-a good choice for low-light viewing.

binoculars exit pupil
Binocular Exit Pupil, Image credit(Takelot.com)

The wider the diameter of the exit pupil, the more light that can pass through, resulting in brighter, easier-to-see images when lighting is poor. If you anticipate regularly using binoculars in low-light situations-at dawn, dusk, within dense tree cover or while observing the night sky-seek out models with a high exit pupil number, preferably 4mm or higher.

Binoculars Field of View

The binocular field of view is often expressed as the width in feet at 1,000 yards, or in degrees of field. this determines the area of the sky or land which is seen through your binoculars and this number is determined by the design of the optics used in the device.

When the field of view is expressed in degrees is termed angular but if it is expressed in feet or meters is called a linear field of view.

As the definition indicates, which means that the wider the field of view of the binoculars, the larger the area of the object or whatever you are viewing.

Focus Distance

This figure may not be important to many but to the few that really appreciate little details, it will be important to them.

Many bird watchers and insect viewers like to have a close minimum focus distance that can allow them to see a minute detail of birds—like wing bars, beak shape, or crown markings.

In this type of cases, the shorter the focal distance of the binoculars, the more details you can see.

The minimum close focusing distance of the binoculars is about 1 to 2 meter. This distance varies from about 0.5 m to 30 m, depending upon the design of the binoculars.

Binoculars Focus

There are three binoculars focusing types used. These are Auto-focus(self-focus), center focus and individual focus.

The self or auto-focusing type are designed such that they have no focusing mechanism in them and have real depth of view 9(usually from around 40ft to infinity).

Their optics are made so that you can use your natural eye’s ability to focus. What this means is that anything from 40feet to infinity will be in focus. People with eyes problems, the center focus is not usually recommended.

The center focus binoculars are the most common types. These ones you will find a central knob in between the two oculars and is adjusted symmetrically. With this type of focusing, the manufacturer usually has a dioptric adjustment on one of the eyepieces to help fine-tune the focus to match individual needs.

Certain models have the dioptric correction integrated into the center focusing mechanism.

For individual focus, each eyepiece has the ability to focus independently.  This allows for precise focusing and ideal if you are using the binoculars alone. This is often used in astronomical and marine models.

Water and Fog Proof

Every outdoor activity you embark on introduces specific data in the environment. Environmental conditions such as moisture, rain, dust, snow or fog can affect the images you are viewing.

These days, you will find binoculars that are not weatherproof and those that are water and fog proof. The ones that are not waterproof should not be used at sea or in a moist environment. Otherwise, they will develop mold and fog inside.

The fully weatherproof binoculars are ideal for any environment.  When a pair of binoculars is waterproof, it means that it is O-ring sealed to prevent any moisture from entering it. But it can still fog up.

Fog proof is when the inside of the binoculars are filled with chemicals such as nitrogen or argon to prevent moisture from condensing inside the binoculars. Otherwise, they will fog when air that contains moisture enters them.

When a binocular is fully weatherproof, it means that it is both water and fog proofed.

Note: All fog-proof binoculars are waterproof, but NOT all waterproof binoculars are fog-proof.

Lens Coating

Lenses are usually coated for the primary reason of reducing glare and reflections that happens when the lights travel from the air to the lens. This coating increases light transmission and contrast.

This is done to ensure that the images produced are very vivid. Lens coating, in general, is good. You will find different coatings are used on binoculars. There are partial coating, fully coated, fully multi-coated lenses. The more layers of coatings that are applied to the lens, the better the quality of images it will produce.

There are some manufacturers who just apply cheap coatings to lenses but which in infect does nothing to the quality of the image. Just do your research and read user reviews of the product before purchasing.

A classic lens-coating material is magnesium fluoride, which reduces reflected light from 5% to 1%. Modern lens coatings consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors.

Prism Coating

There are two popular types of prisms used in binoculars. These are roof prisms and Porro prisms. The prism type used in the binocular is used to describe its type.

A prism is just a large piece of glass used inside the binoculars to correct the image orientation created by the lenses. They also help in making binoculars small in design by shortening the light path.

Binoculars that have both lenses and prisms coated are usually of high quality. When the prisms are coated, maximum light gathering or collection takes place. thereby improving the image brightness and crispness.

The upper echelon of prism coatings is called dielectric coatings, which allow almost 100% of the light through the prism.

Another type of prism coating, only used on roof prisms, is called “phase-correcting” coating. Because of the way roof prisms reflect light after it moves through the objective lens, it gets split into two separate beams that travel through the prism system independently.

The beams experience a “phase shift” as one beam strikes the eyepiece lens a fraction of a second before the second beam. When the two beams are recombined in the eyepiece lens they are slightly out of phase with each other, which can affect color balance.

By applying special coatings on the prism, the faster light beam is slowed to match the slower beam, bringing them back into a phase when they hit the eyepiece lens. Thus, greatly improving color, clarity, and contrast.

Specialty Binoculars

These are the kinds of binoculars that employ special techniques and technologies in their designs. Such as rangefinder and image stabilized.

Image Stabilized

Image stabilized binoculars use an image stabilization technology in high powered binoculars to create image stability. The two types of techniques used in image stabilizing are; gyroscopic and electronic stabilization.

This is achieved using an internal gyroscopic mechanism or with electronic sensors coupled to microprocessors that instantaneously adjust for any motion.

These techniques allow binoculars up to 20× to be hand-held and much improve the image stability of lower-power instruments. Image stabilized binoculars tend to be more expensive and heavier than similarly specified non-stabilised binoculars.


Rangefinder binoculars typically display the distance to the target in either feet or meters, with the readout visible in the eyepieces.

These kinds of binoculars are designed with infrared (IR) laser integrated that is used to measure the distance from the binocular to an object.

They can be used at sea to measure the distance to another ship or possibly someone who needs rescuing, help hunters to measure the distance to their target, or aid golfers to calculate their swing to the green.

The latest versions incorporate an inclinometer that measures the uphill or downhill angle from you to the subject.

So, as you can is clearly understanding binoculars specifications is essential to select the right kind of binoculars for your needs.


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