June 23rd, 2021

By A.J. Gentile

Would you like to observe wildlife on your next hike with a good pair of binoculars, but you don't know which one to choose? In this article, our expert A.J. Gentile will introduce you to the different aspects to consider when buying your binoculars.


What do the numbers on the binoculars mean?

The numbers on the binoculars (ex. 8x42, 10x42) refers to both the magnification and diameter of the objective lens (the first lens elements) where the light enters the binocular.

The magnification is represented by the first number (8x, 10x or more) and it stands for how many times the image is enlarged from our natural vision. While the second number (32, 42, 50 and more) represents the diameter of the objective lens in millimeters, also known as the aperture. The larger the number, the more light the binoculars can allow in. It is these 2 numbers that determine the best use of the binoculars.
PRO TIP: The larger the aperture the more the binoculars can be used in low light situations.

Why do binoculars with similar specifications cost more than others?

The cost of binoculars all depends on their construction and materials used such as the kind of glass used or what kind of micro coating were applied to the surfaces of the glass elements. ED glass, known as Extra Low Dispersion glass, helps to minimize the problem of chromatic aberration (mismatch at the focal point where the colors do not combine as they should).

There are also 2 types of glass used for prisms in binoculars. The more popular one is the BK7 glass which is ordinary crown glass, while a far more superior prism is made of BAK-4 barium crown glass. It is a more expensive prism which will give you much more contrast and image sharpness.

What other additional features are worth considering when buying binoculars?

Are they impact resistant? Are the binoculars sealed against weather, water and/or fog? Binoculars that are fog proof while being sealed at the same time, are usually filled with an inert gas to replace the air inside and not fog up while the ambient temperature fluctuates.

How do I choose a pair of binoculars for my specific use?

For backpacking, hiking or theatre

Binoculars used for hiking, backpacking and what we call theatre glasses, are preferable to be as compact as possible. Their magnification is no more than 8x or 10x and usually have a diameter of less than 30 mm.

For wildlife
Binoculars used for wildlife, including birding and whale watching can be slightly more robust and can be of the roof prism or porro prism type binocular (see below for prism comparison). The preferred magnification for wildlife watching will usually be between 8x and 12x max, and the diameters are usually between 42 and 50 mm.

My personal preference is the 8x42 of the roof prism type, as anything over 8x for me is too shaky and I find the porro prism to be too bulky.

Marine Binoculars
Marine binoculars used for navigating usually start at 7x or 8x with diameters of 50mm or 60 mm. Many marine binoculars are of the porro prism type because their more robust construction can include an optical stabilizing system and compass readouts for navigation.

For Astronomy
Most binoculars for astronomy will have magnifications of 9x, 20x, 25x with diameters of 60 mm and more. Binoculars of sizes like 20x80 or 25x100 will need to be used on a tripod as these are just too heavy to hand hold.

Understanding the specifics of the Binocular

Binocular Size

Binoculars usually come in three sizes, compact, mid-size and full-size. Your preference should be guided by what you want to be observing and what you want to be carrying around with you.


Compact binoculars
These usually have a principal aperture of 30 mm or less and use the Galilean optics or roof prism type.

These will have apertures varying from 30-42 mm and be of the roof prism type.

These include everything over 50 mm in diameter and are all in the realm of the porro prism.

Objective Lens diameter

The objective lens diameter (ex. 8x (25, 30, 42, 50+)) are key to how much light can enter the binocular. The more light that can be brought to the eye, the more details can be observed in dim light. The principal reason why I want a large aperture for Astronomy is because we are observing the night sky.

Magnification Power

The magnification value is what allows us to view our subject closeup and in detail. Usually, the preference of magnification is 8x or 10x, as anything above that will increase our own handheld shake, and anything that might be moving in our field of view between the subject and us.
Binoculars of 8x25, 8x42, and 8x50 will all give us the same magnification but will have a large effect on field of view.

Field of view

The field of view is how wide of an area is the binocular's image covering which can be expressed as an angle (ex. 8°), or as the width in feet at 1000 yards (420 ft). To convert the angular field to linear field, multiply the angular field by 52.5.

Field of view is a matter of eyepiece design. More power means a narrower field of view, while wide field of view eyepieces have a shorter eye relief and may not work with eyeglasses.


Exit Pupil

Exit pupils are the small, bright circles seen in the eyepieces when holding the binoculars away from the eyes and up to the light. They are the actual beams of light coming out of the eyepieces entering the eyes.

The exit pupil is calculated by dividing the diameter of the objective lenses by the power of magnification. An 8x42 binocular has an exit pupil of 5.25 mm (42/8=5.25) while a 10x42 has an exit pupil of 4.2 mm (42/10=4.2).

In the middle of the day, the pupils of our eyes contract to between 2 and 4 mm and at night they may open to 7 mm. If the beam of light entering the eye is wider than the pupil of the eye, the excess does not get in and the eye can't see it. During the day things look just as bright in binoculars with a 4 mm exit pupil, as with those with a 7 mm exit pupil.

The larger exit pupil, the more advantage we have in low light. For astronomy, an exit pupil of 7 mm is standard, while for birding and wildlife, an exit pupil of 6 mm is usually large enough for the most demanding low-light situations.

As we age, the eye loses its ability to adapt to low-light conditions. While a young adult will have a pupil diameter of 7 mm, at 50 years old, the pupils may only open up to 5 mm. Therefore, the binoculars with the larger exit pupils may not help the older birder.

Eye Relief

Eye relief is simply the distance from the outer surface of the eyepiece lens to the position where the exit pupil is formed, also known as the eyepoint. When looking through the binoculars from the eyepoint, you can obtain the whole field of view without any vignetting (a reduction of an image's brightness or saturation toward the periphery compared to the image center).

For those wearing eyeglasses, it is recommended to use binoculars with a longer eye relief or a high eyepoint. It is recommended to use a pair of binoculars with an eye relief of at least 16 mm, if not more. On some binoculars, even with the eyecups turned down or retracted, the glasses aren't close enough to the eyepoint therefore the observer is losing much of the field of view. 

Understanding binocular optics

Prism type

Prism binoculars use prisms in the optical system in order to rectify an inverted image projected by the objective. The prism also shortens the optical path, resulting in a compact body. There are two types of prism binoculars: Roof (Dach) prism type and porro prism type.


Porro Prism
The porro prism was invented by Ignazio Porro in mid-19th-century Italy. All its reflective surfaces are completely reflective, so it loses no light, and such binoculars are easy to produce. However, the optical path is bent like the letter Z using 4 reflections and thus no loss of light. Accordingly, this prism system takes up considerable space, so that binoculars with a porro prism are larger than those with a roof (Dach) prism.

Roof Prism
The roof prism system is used to rectify the image for this type of binoculars. "Dach" means roof in German, and this type of prism features a roof-shaped surface. The optical path at the objective side and eyepiece side is virtually straight using 6 reflections and hence a slight loss of light and making it possible for the binoculars to be compact and lightweight. However, manufacturing and adjusting the prisms requires very highly precise technology. Demand for this type of binoculars is expected to grow as more customers prefer its slim, stylish design.

Protective features for binoculars

Waterproof vs Weatherproof

Waterproof binoculars will have a sealed casing that will be completely waterproof for a certain depth and for a certain amount of time. They will normally be rated using the JIS waterproof scale, such as JIS Class 8: The device remains dry during continuous immersion in water.
Weatherproof, weather-protected, or weather-resistant binoculars are most certainly not waterproof at any depth for any amount of time. They do however have some resistance to water and thus should survive light rain, but are not intended to be fully, or even partially immersed in water. Please note that because these water-resistant products are not fully sealed, they are also not fog proof in any way.

Fog-proof binoculars

Binoculars that are Fog-proof are most definitely sealed casings but the air (O2) in the optical barrels of the binoculars have been completely purged and replaced with an inert gas such as Nitrogen or Argon.
Both gases are considered dry gasses and Argon is said to have a larger molecule than Nitrogen and therefore reacts less with temperature change. Also having Nitrogen or Argon replacing air in the binoculars means that there is virtually no chance of mold or fungus forming on the lens elements inside the optical system.

Rubberized casing

Binoculars with a rubberized casing, will be protected against any light to medium impact damage.

Techniques using binoculars

Testing binoculars

Most specialized nature related or photography retailers selling binoculars, will have a resolution chart (shown below) placed at least at a distance of 20-50 feet away for you to use when trying and comparing different makes and models of binoculars. Remember when testing binoculars to always adjust the diopter for the sharpest focus possible and compare the indicated sizes of lines on the resolution target.



Focusing with binoculars

  • First start by adjusting the eyecups by either extracting the eyecups for those without eyeglasses or retracting the eyecups if you wear glasses.
  • Next, looking through the left eyepiece of the binocular, focus with the center focusing dial.
  • Once the image in the left eyepiece is sharp, close your left eye.
  • With your right eye in the eyepiece looking at the same image, turn the diopter ring on the right eyepiece until the image is at its sharpest.
  • Once you have adjusted your diopter for your right eye. You have adjusted for the difference between focus of both eyes. You may now use the center focusing ring to adjust your focus.




Cleaning binoculars

Cleaning the outer casing of the binoculars with a damp microfiber cloth is always good practice. As for the objective lenses and eyepieces, using a microfiber cloth with liquid lens cleaner (nothing with ammonia), or humid lens or glasses tissues are preferred for cleaning the optics.

Now you have everything you need to choose your binoculars properly and use them to their full potential. Good observation!

Steve Jackson