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How To Choose The Right Binoculars

How To Choose The Right Binoculars

AstroTelescopium Team |

Many amateur astronomers believe that the telescope is the optical tool needed to sufficiently observe the night sky. 

However, sometimes you need to walk before you can run.

In this case, we're referring to the gradual evolution from naked eye observations to binocular use, prior to any sort of celestial viewing with a telescope.

Convenience, versatility, field of view, and power are a few key benefits that make binoculars a worthy upgrade from naked eye viewing before the transition to a telescope.

Additionally, lower cost is an important consideration if you are debating between purchasing a new pair of binoculars versus a new telescope.

In this guide, we'll cover:


Binocular terms

Before we dive into binocular terminology, let us begin with a clear definition of what binoculars actually are. Binoculars are optical instruments that combine a pair of telescopes (monoculars) in order to view distant objects through both eyes.

Bresser Binoculars

Magnification and Aperture

The first thing you will typically notice when browsing through the multitude of binocular types are the two numbers written in a form such as: "10x42". The first number references the binocular's magnification and the second number references the binocular's aperture. Magnification, or power, is the factor by which to enlarge an object and make it appear closer. Aperture is the diameter of the objective lens, measured in millimeters. Therefore, the "10x42" example explains that this particular pair of binoculars has a magnitude of 10x and a lens diameter of 42mm.


There are two main types of binocular prisms in use today: the Porro prism and the Roof prism. Traditionally designed binoculars are manufactured with the N-shaped Porro prism. The Roof prism's straight tube design is more costly to manufacture but they are far more popular in modern times. Binocular prisms have the key purpose of folding the optical light path to alter the orientation of an image. The Porro prism was named after its inventor, optician Ignazio Porro. In binoculars, the porro prism models use external focusing. Alternatively, binoculars utilizing the roof prism have an internal focusing system.

Field of view - Apparent vs Actual

The field of view refers to the visible range of a particular position, usually expressed as an angular area measured in degrees. Apparent field of view is the angular diameter of the image you can see through your binoculars. It is solely a property of the eyepiece design. The wider the apparent field of view, the greater the amount of sky that can be observed. The narrower the field of view, the more limited your observing experience will be.

In order to figure out the actual field of view, the following formula is used:

Actual FOV = Apparent FOV / Power

A brief example will make this easier to understand. Let's say you have a pair of binoculars with a 60-degree apparent field of view and a power of 10x. In this situation, the actual field of view will be 6° (60 divided by 10) of visible sky.

Exit Pupil

The exit pupil is a term that refers to the size of the light cone exiting the binocular's eyepieces. Measured in millimeters, the exit pupil is calculated by dividing the aperture by the power. Therefore, a pair of 10x50 binoculars would have exit pupils of 5mm (50 divided by 10).

As a rule of thumb, the binocular's exit pupil should match the dilated pupil of your eye. That number is approximately 7mm. However, after age 30, we lose roughly 1mm every 20 years due to eye muscles losing their flexibility. Therefore, a 25 year old and a 65 year old may need to review the exit pupil in binoculars differently. While the 7mm may work well for the 25 year old, an exit pupil of 5mm will be more beneficial for the 65 year old.


How to use binoculars for astronomy

Without any special equipment, an observer can see approximately 4,000 stars with the naked eye. The use of binoculars increases this observational ability to over 100,000 stars.

For astronomy viewing you should, at the minimum, use a pair of binoculars with a magnitude in the 7x to 10x range and an aperture in the 42mm to 56mm range (we covered magnitude and aperture in the previous section).

The first step to take before utilizing binoculars for celestial viewing is to get them focused. Find a bright star in the sky, aim your binoculars towards it, and hold it centered in your field of view. Then, look through the left eyepiece of the binoculars and turn the focus wheel until the star is as bright and clear as possible. Once that is set, look through the right eyepiece and set the diopter adjustment. Again, making sure the image looks as clear as possible.

Explore Scientific Binoculars

When using binoculars for celestial viewing, if you are focusing on a specific section of the sky, it may help to mount your binoculars on a tripod. This prevents you from having to hold the binoculars in place for an extended period of time, which can become tiresome.

This is a good place to interject the consideration of weight vs aperture size. Although binoculars with larger apertures can gather more light and provide brighter images, the extra weight can make them more difficult to hold steadily during longer observation sessions.

For an optimal stargazing experience, you will want to travel away from the bright city lights to reduce any obstructive views. Through a pair of binoculars, bright stars can appear with great color contrast. However, similar to telescope views, you shouldn't expect distant galaxies and nebulas to appear overly vivid.


Binoculars vs Telescopes

As we mentioned in the introduction, binoculars are often underrated to many amateur astronomers. Especially those just beginning their journey into astronomical observations. However, we do not blame them, since the majority of attention and marketing is driven towards telescopes when conducting search queries like "how to view the stars", etc.

Vixen Binoculars

Here are 4 points that speak to the advantages of binoculars over telescopes:

  • Easy and intuitive. Unlike telescopes, there is a much shorter learning curve in order to get the most out of your binoculars. Simply hold them up to your eyes and look through them. Binoculars do not have a complicated set up that many types of telescopes require before use. Additionally, images appear right side up similar to naked eye viewing. Opposed to telescope images that often appear flipped or upside-down due to the arrangement of mirrors and lenses.

  • Greater portability. Binoculars typically weigh much less than telescopes, are significantly smaller in size, and are much more convenient to carry around. On the other hand, telescopes require additional accessories (eyepieces, mounts, etc), which make them less convenient to carry around.

  • 2 eyes are better than 1. Binocular, or two-eyed vision, provides better depth and distance perception versus single-eye vision. This has much to do with the data processing that occurs in your brain when viewing something from two different angles. Since binoculars are naturally constructed to be used with both eyes, observers can see greater detail of the distant objects they are viewing.

  • Wider field of view. Compared to telescopes, binoculars show observers a much greater portion of the sky. They allow you to see entire constellations in one frame. A typical pair of binoculars will portray roughly 7° of sky while a typical telescope will portray about 1° of sky. This wider field of view allows for easier searching of celestial objects during observation sessions. For example, "star hopping", a technique used in astronomy for finding fainter celestial objects through the guide of brighter stars, is much more difficult with telescopes.

Our intention here is not to dissuade you from purchasing a telescope in the future. We simply don't want you to miss out on the essential uses of binoculars and their ability to help you learn some basic observational skills and simplify the layout of the sky.


Start observing

You should now have a better understanding of key binocular terminology, be more familiar with how to use binoculars for astronomy viewing, and know why binoculars are an ideal optical tool to use prior to telescope observations.

If you are in the market for purchasing a high quality pair of binoculars, AstroTelescopium has a curated selection of binoculars from industry-leading brands that can help you explore the night sky.

On the other hand, if you think you're ready for the larger optical tool, be sure to first read how to choose the right telescope.



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