What Is a Zoom Lens on a Monocular? – All You Need to Know

People often ask, what is a zoom lens on a monocular?

Let’s find out!

What Is a Zoom Lens on a Monocular?

A zoom lens is a type of optical system that allows you to adjust the focal length. Monoculars with a zoom lens can’t be focused at different ranges; however, they’re still useful for close-ups and long-distance viewing.

Binocular zoom lenses are usually used in binocular telescopes because they let users see more of their surroundings without having to adjust the binocular telescope.

Monoculars and Binoculars

A monocular is an optical instrument used for viewing distant objects.

Binoculars and monoculars are tools used for observing objects from a certain range. Like any other tools, they have their own units of measure and technical words that may seem a bit hard to understand at first. Actually, it’s quite simple.

Whether it is for fun, to cultivate a new interest or gather more info before buying, I am sure that you’ve wondered “what do the numbers that show up on the monocular mean?”

There are different types of lenses available such as 12x50mm, 10x42mm, and 7x35mm, but what do these numbers mean?

You will be able to understand the meanings of the numbers that appear in the monocular or binoculars, which models are best suited for your needs, what are the important characteristics to look out for when choosing a pair of binocular, and useful hints for professional observations.

What do these numbers mean?

We can distinguish between different types of monoculars and/or binoculars by using two digits: for example, 12x50mm or 10x42mm.

The first number (called “magnification”) tells us how many times larger an object appears when we look at it through a telescope. For example, if we see something through a 10x42mm lens, then we’re seeing it 10 times bigger than we would normally see it.

The second number after “x” is in millimeters and represents the size of the objective lenses. A 12x50monocular has a 50-millimeter (or 0.5 inch) objective, while a 10x40monocular has a 40-millimeter (or 1.6 inches) objective.

Lens Diameter

The second number after “mm” indicates the diameter of the object lens expressed in millimetres which determines the size of the image formed by the camera sensor. This feature is important for low light conditions such as at night or when there is not enough ambient light.

The bigger the diameter of the lenses = The brighter and clearer the images = The heavier the binoculars

Larger lenses mean heavier monoculars. Monoculars with a maximum lens size of 50 millimeters are usually recommended for traveling, excursions, and outdoor activities. For trekking and daytime observation, a lens size of 20 millimeters is sufficient; however, for nighttime observation, a lens size between 50 and 60 millimeters is necessary.


Magnification is the number one thing to look for when buying binoculars. It tells us how close we’re going to be able to see things.

The bigger the zoom (magnifying glass) = The narrower the angle of view.

What does that mean?

If we look at an object through a microscope, the size of the object seems larger to us because our eye has a smaller field of vision. However, if we observe the same object using binoculars, the field of vision is wider. Therefore, when looking at objects with a magnification greater than 10x, we need to use a tripod so that the camera remains steady and the images remain sharp.

If you’re looking for a wide view and want to see everything at once, use a low power setting.

what is a zoom lens on a monocular

What Is a Zoom Lens on a Monocular?

Field Of View (FOV)

The field of vision represents the range of angles from which an object can be observed at a distance of 1 kilometer. It can be measured either in meters or in degrees.

Let’s make an example.

The technical description of our monocle says that at a distance of one kilometer, we will be able to see a 120-meter-wide visual angle.

The visual angle refers to the area that we see through our eyes when looking at something. It is calculated using a mathematical equation where the number 17.5 represents the distance between the eye and the object in meters corresponding to an angle of one degree.

A visual angle of 1,000 meters equals 17,500 degrees.

Field of View (FOV) = 1,000 meters / 17.5 degrees

We’ll now return to our example: 12x50mm monocular with 120M/1000M FOV and use both formulas to calculate the FOV and the AoV.

Field of View (FOV) 1000M = 6.85×17.5=120

A visual field of 6.8 degrees equals one eye.

A monocular camera has a field of view (FOV) of 120 million meters per thousand meters (120 M/1000 m), and an angle of view (AoV) of six point eight degrees.

A wider field of vision means less magnification. For example, a 10× monocular has a larger field of vision than a 12× monocular.

A wider field of view means greater magnifi­cation.

Exit Pupil

When considering purchasing a monocular, another important factor to take into account is its entrance pupils expressed in millimeters which indicate the size of the lens aperture through which light enters the eyepiece.

Exit Pupil = Lens diameter / Magnification

Let’s return to our 12×50 binoculars, the exit pupil diameter will be 4.5 inches (50/12). Common values for daytime and night vision imaging.

Let’s figure out why.

We should take into account the size of the exit pupil because it should be at least equal to the diameter of our pupils, which go from 2 millimeters in bright light environments to 7 millimeters in low light conditions.

The larger the exit pupil (the distance from the lens to the retina) is, the greater the transition of light (from the lens to the retina), which means the brighter the image

Now everything is clear! For nighttime observations, it all comes down to the values shown in the products’ descriptions. Next, when you read the “exiting pupil” value, you can get some ideas about whether the telescope is also suited for twilight or sunset observations too.

Eye Relief

Exit pupils go from 3 to 9 mm, but eye relief ranges from 5 to 20 mm.

Knowing this value will help you understand why you should be doing something.

It is particularly important for people who use eyeglasses to understand whether the lenses can be used together. If the space between the eyes and the lenses is greater than 17 millimeters, then even when using eyeglasses, the monocular will display the same field of view as without them.

The longer the eye-relieving distance = The better the observation without glasses.

For people wearing eyeglasses, the ideal eye relief is 20 millimeters (or 2/3rds of an inch) and is also perfect for observing clearly and blinking.

For example, our binoculars have an eyepiece with a 20mm diameter and through the diopter wheel, you can change the focal length according to your prescription.

Types Of Lens

Monoculars and binocular lenses can be made from any number of materials including glass, plastic, and even wood.


They are used for their high strength and low weight. Images appearing in the eye are of lower quality, therefore, they’re less expensive.


They are mostly used for professional monoculars and binoculars. These devices reflect the light coming from the outside, thus producing high-quality pictures. However, they also minimize the reflection of light, so that the image is not distorted.

To do this, lenses are treated with special coatings that prevent reflections. In addition, abbreviations are used to identify the type of coating applied to each lens.


C (Coated) external lenses have been coated with one layer of anti-reflective coating.

MMC (multi-coated) external lens: External lens has been coated with multiple layers to ensure a better light transmittance.

Fully Coated: Both the internal prismatic lens and the external lenses have an anti-reflection coating applied to them.

Multi-fully coated: both the internal prism and the external lens have been coated with multiple anti-reflection coatings for better light transmittance and greater protection against water, humidity, scratching, and accidental falls.

Multilayer FC and MFC coat­ing is better than a single layer coating and this also affects the price of the monoculars and the binoculars.


We hope this article was helpful to you. Let us know if you have any questions, in the comments below. 


  • John Moses

    John is the Editor in Chief here at The Outdoor Stores. His area of expertise ensures that there is no one better to suggest which rifles are most suitable for your hunting experience. He is also available for you to contact him personally to discuss the types of animals you want to hunt and the terrain you will be hunting on. Feel free to read his posts for expert opinion on Rifles, Scopes, Rangefinders, Bonoculars and Monoculars.

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