All About Binoculars – How To Choose The Best Binocular

All About Binoculars – How To Choose The Best Binocular

If you have decided to buy binoculars and have never done it before, there is a good chance that you will be overwhelmed or confused by the technical information. Binoculars are a rather complex optical instrument, so there are many things to clarify and explain. In this article we’ll discuss all of the parts in a pair of binocular and the binocular terminology.

Choosing The Right Binoculars

Before deciding on the binoculars you want, you need to consider what you’ll be using them for. It is essential to know the “combination” suitable for your use. That is referring to the technical characteristics that are indicated together: the number of enlargements (i.e. how many times an object is seen larger than the actual reality), and the diameter of the larger lenses (i.e. the two objectives located outside the binoculars).

Recognizing this combination is extremely easy, because inserted next to the name of the specific model. For example, seeing 10 × 42 means that the binoculars in question are able to magnify ten times and offers objectives with a diameter 42 mm. For more information on how to choose the best binoculars for your needs check out our last article, “Best binoculars for bird watching”, here on Avibirds.

1. Types Of Prisms

A binocular’s good image quality is determined by which kind of prisms it mounts, by the treatments applied and by the materials used. The light that passes through the lens of the binoculars creates an image that reaches “upside down” to the eye of the beholder, and which must therefore be “straightened”. How? Precisely through the prisms, of which two types are distinguished: the Porro prisms and the roof prisms.

Binocular types of prisms

Porro prism binoculars are named after their inventor. They constitute the simplest construction type, and therefore the cheapest. From the lower number of refractions of the image, however, they have the disadvantage of weight, since they are two rather “massive” adjacent prisms.

Roof prism binoculars are smaller, more robust and compact than the porro prism binoculars. The roof prisms – so defined due to their characteristic shape. One of the edges requires greater manufacturing complexity. That edge is provided with more surfaces, which require absolute precision, avoid affecting the clarity of the image. This explains their generally higher cost compared to Porro prisms.

2. Coatings And Materials Used

A small part of the reflected light beam is dispersed every time it “comes into contact” with a surface (even if made of glass). This requires the glass to have a low refractive index. It also means that the surfaces “crossed” by the light beam itself are characterized by specific coatings that reduce its dispersion. In this case we speak of “coating” “conjugated” in different ways based on the number of layers applied and the number of surfaces: Coated (C), Multi Coated (MC), Fully Coated (FC), Multi Fully Coated (MFC).

Binocular coating and materials

As for the materials, which are also “competitors” in determining the quality of the image, we refer, as can be easily understood, essentially to glass. There are generally two types of glass: the BaK4 and the BK7. Let’s briefly examine their characteristics and differences.

BaK4: crown-barium glass, has a low refractive index and low light scattering, characteristics that make it widely used in the manufacturing of optical groups of the best binoculars.

BK7: borosilicate glass with great resistance and transparency, it is particularly suitable for light transmission.

3. Brightness

The brightness transmitted by the binoculars is determined by two factors:

  • The amount of light that “enters” from the lens and what “leaves” from the eyepieces.
  • The width of the field of view depend on the combination of magnification / diameter of the objective lenses.

Focusing here on brightness, we underline that it is indicated with the expressions “brightness index” or “exit pupil”: which is the value, in millimeters, that expresses the diameter of the light beam coming out of the binocular eyepiece. In other words the useful space for the pupil of the eye to see through the instrument. The higher this value, the greater the brightness of the binoculars, i.e. the ability to pick up light.

Binocular brightness

To observe the outgoing pupil look through the binoculars against some light and look through it from the eyepiece side at about 30 cm distance: on both eyepiece you should see a luminous ring created by the light going out. To obtain the exit pupil diameter you need to look at the ratio between the amount of magnification and the lenses’ diameter. (For example, 8 × 30 binoculars mean that they have an exit pupil of 3,75 mm).

It is important to know the brightness index of the instrument in question as it will affect the possibility of using this instrument exclusively during the day, with full light. Even in the case of low light (at sunrise or sunset): the diameter of the pupil, in output it is proportional to that of the human pupil, but variable according to the light.

4. Binocular Field Of View And Magnification

Inbuilt in the number of magnifications made by the binoculars, the visual field indicated the “measure” of the distance that the binoculars can be used to observe the ground. The measurement can be expressed in meters or degrees. For meters, the length of the perceptible portion is identified from a distance of 1000 m (for example 110 m). Looking at degrees, the amplitude of the angle of the visual cone “circumscribed” by the binoculars is indicated (for example 5 °).

Binocular field of view

The greater the number of enlargements carried out by the device, the smaller the part of the ground that can be observed. Likewise, the opposite is true – the lower the aforementioned number, the more extensive is that part that the device allows to “grasp”.

If your need is for example to observe animals in a “detailed” way, then you will need a fair number of enlargements; whereas the observation of moving animals within a rather wide view will require a smaller number of magnifications. Everything, therefore, is strictly linked to personal, specific needs.

5. Comfort

It is important to be able to adapt the binoculars to the shape of your face and your eyesight, especially if you are using binoculars with glasses. This is why they should provide certain adjustments that allow them to be used easily by any individual. We consider them briefly below.

Focus. Reached by means of a special wheel, it aims to display the object to be observed with the greatest possible clarity. Mostly central – that is, the aforementioned wheel is located between the two tubes. It can also be independent, in the sense that each eyepiece is equipped with an adjustment ring, and the focus is managed independently.

Binocular comfort of use

Eye relief definition. It is a dual function adjustment, since it allows, on the one hand, to place the binoculars on the face, and, on the other, to make the outgoing pupil “coincide” with the observer’s pupil. It is obtained by means of two soft “cups” placed on the eyepieces, which can be extracted and adjusted according to the desired length. We distinguish the removable cup, with the greater possibility of customization, and the folding one, which is less flexible since the cups can be folded in only two positions.

Dioptric adjustment. Important to focus the image correctly with both eyes, it is often present even in low-end models. Generally, the ring for this adjustment is located on the right eyepiece.

Inter-pupillary distance. This is the measure of the distance between one center of the pupil and the other. For a correct use of the instrument under examination it is necessary that the eyepieces of the same can adapt perfectly to their pupils.

6. Solidity And Design

The term solidity refers to those structural characteristics that make binoculars an instrument capable of enduring any atmospheric agents or falls, or contact with dirt and water.

Waterproofing. It represents the minimum for a good product, because otherwise dust or water would enter the binocular tubes, compromising their vision.

Binocular solidity and design

Coating. To cope with possible blows or accidental falls, it is of fundamental importance that the pipes have a good coating, such as soft rubber. A material that, among other things, is also anti-slip, to allow a firm grip in case of slightly sweaty hands or in case of rain.

Filling. It is important to check, at the time of purchase, that the binocular filling is anti-condensation. In the presence of strong temperature variations, when binoculars are used outdoors – especially if they are filled with air – internal condensation can occur. Some binoculars are filled with gases which are not subject to condensation, like nitrogen.


With so many binoculars on the market, it can be a rather difficult task to choose the best set for your needs. We believe that this article will be of great help to you. Keep following us on Avibirds to know more about the best birdwatching equipment and how to choose the best binoculars for birdwatching.