Choosing the right binoculars — tips from practitioners

Choosing binoculars is not an easy task. Especially if you have never been interested in optical technology until this very moment and have no idea what all these numerous abbreviations and numbers on the packaging of the device mean. Where to begin? Which model to choose? What to look for before buying? Let’s try to figure it out together.

What are binoculars?

Binoculars are a complex optical device that is designed to monitor objects distant from a person. It consists of two optical tubes located parallel to each other. The tubes are interconnected, which allows the observer to obtain a stereoscopic image, which is not available when using a monocular instead of binoculars.

The design of binoculars includes three main elements:

• eyepiece (part of an optical instrument facing the eye of the beholder);
• lens (lens or lens system directed at the object of observation);
• prism (with its help, light is refracted and the image is flipped).

Features of the design of optical prisms

The prism is one of the most important elements of binoculars. Image quality, reliability and even the appearance of the device largely depend on its properties.

Two types of optical prisms are used in modern binoculars: Porro and Roof. The former are distinguished by high plasticity, i.e., they convey well the terrain and the depth of space. Models with Porro prisms have a characteristic curved shape: the distance between the lenses is greater than between the eyepieces. Due to this, it is possible to use lenses of increased diameter, which is especially important for astronomical and marine binoculars.

Roof prisms allow you to position the lens and eyepiece on the same axis. The contrast and brightness of the image in such models is lower than in binoculars with Porro prisms. These shortcomings are compensated by the low weight and high compactness of the device. Roof prism binoculars are an almost perfect choice for the outdoors.

Expert advice! For use in the field, purchase binoculars whose weight does not exceed 1 kg. Heavier models will not be very convenient to carry around all the time.

Specifications of binoculars

The main technical characteristics of binoculars are usually indicated by the manufacturer either directly on the body of the device, or in a brochure or instruction manual enclosed inside the box.

multiplicity

Magnification is the degree to which an object is magnified when viewed. Denoted by the symbol «X». If the packaging says 12X, this means that the binoculars are capable of magnifying a linear image by 12x.

In practice, 6-8x magnification is optimal for field models. In exceptional cases, binoculars with 10-12x magnification are used. With a further increase in the magnification, the observation deteriorates due to image jitter, and one has to use a tripod.

Aperture

Aperture is the diameter of the lens of the device. Designated in millimeters. It is usually indicated in the accompanying documentation immediately after the multiplicity. It looks something like this: «10X50» (10x magnification with a diameter of 50 mm).

Depending on the aperture, all optical binoculars are divided into the following groups:

• standard (from 30 to 50 mm). Designed mainly for land and sea observations;
• astronomical (from 50 to 80 mm). As the name implies, they are made to study astronomical objects. Unsuitable for ground observation;
• large astronomical binoculars (from 90 mm and above). Their characteristics are close to telescopes. Due to their large dimensions and weight, they are practically not used without a special tripod.

Exit pupil diameter

The exit pupil, or exit pupil diameter, is the width of the light beam that is directed from the eyepieces to the observer’s eye. It is calculated as the ratio of aperture to magnification. That is, a device with a magnification of 8X and an aperture of 40 mm will have an exit pupil diameter of 5 mm.

During the day, the size of the pupil of the human eye changes (from 2 to 8 mm). The diameter of the «pupil of binoculars», on the contrary, remains constant. For comfortable viewing, the exit pupil of binoculars should be approximately the same size as the human pupil. Therefore, for long-term use, it is recommended to purchase models with an exit pupil of 5 to 7 mm.

Aperture

This setting controls the brightness of the image. It is calculated by squaring the exit pupil diameter. For binoculars with an exit pupil of 4 mm, the aperture ratio will be 16, for binoculars with a pupil of 7 mm — 49, etc. The larger the aperture ratio, the higher the contrast and brightness of the observed object.

Expert advice! For observation at night and in poor light conditions, binoculars must have an aperture ratio of at least 25 units.

Field of view (true angle of view)

The field of view is the part of space visible through the eyepieces of the binoculars. It is usually measured in degrees (if we are talking about an angular field) or meters (linear field). The higher the magnification, the larger the visible field of view.

A true field of view of 65⁰ or more is considered wide-angle. Binoculars with such characteristics are good for observing moving objects.

Eye relief

The distance from the eyepiece lens to the point on the optical axis at which the pupil of the eye must be placed in order to see a sufficiently clear image. Usually does not exceed 20 mm.

Binoculars with small eye relief (10 to 15 mm) are designed for people who do not wear glasses. Devices with an offset of 15 mm or more — for use in glasses or gas masks.

Expert advice! For observations in difficult conditions, choose shock-resistant models with a rubberized housing.

AR coating

Standard binoculars consist of 6 to 10 lenses. Each lens reflects some of the light (up to 4%) that passes through it. Because of this, not 100% of the light beam reaches the observer’s pupil, but only 45 — 75%.

To reduce reflection, manufacturers cover the optical surfaces of binoculars with a special anti-reflective coating. This allows you to reduce the percentage of light reflection to a minimum. In the technical data sheet of binoculars, the quality and type of such a coating are indicated as follows:

• coated — the coating is applied only to the outer surfaces of the lenses, one layer;
• fully coated — the coating is applied to all optical surfaces, one layer;
• multicoated — some lenses are coated with a multi-layer anti-reflection coating, the rest of the optics are processed in one layer;
• fully multicoated — all optical surfaces of the binoculars are coated with a multilayer coating.

Focusing

It happens central and separate. In the first case, the focusing mechanism, common to both tubes, is located in the area of ​​their hinge connection. With separate focus adjustment, each eyepiece is adjusted separately by the observer using diopter rings.

Central focusing is most often used on amateur or simple hunting binoculars. Separate diopter adjustment is typical for military models.

Additional features and design elements

Some binoculars, due to the specifics of their purpose, are supplied by the manufacturer with additional structural elements that affect the technical characteristics of the device. It can be a laser rangefinder, a compass, a night vision device, a goniometric grid (on military models). Marine binoculars are often equipped with gyroscope stabilizers powered by built-in batteries.

These devices are usually more expensive than their standard counterparts. Whether it is reasonable to overpay for a rangefinder or a gyroscope is up to you. But filling the case with nitrogen or argon is clearly worth spending money on: this will protect the lenses from moisture and save you from problems with eyepiece fogging.