What Do Numbers Mean on Monocular?

Monoculars are used to see objects more clearly and brightly, especially in low-light situations. However, the monocular’s performance is determined by its magnification.

Monoculars are available in a variety of magnification levels to let you view the picture more clearly. Higher magnifications should be used to view small things clearly, whereas lower magnifications can be used for basic goals.

However, a concern comes to our minds: how can we select the appropriate magnification, and is there any material that can be used to verify magnification before purchasing a monocular?

Monoculars are a simple tool that can let you view a lot of what’s in the night sky. With your Monoculars, what can you see? Many features of the moon, planets and their details, and even the brilliance of a comet will be seen.

But what do the numbers on your Monoculars represent, and why do they matter? Here’s your complete guide to understanding your Monoculars and selecting the finest ones for your needs.

Monocular Numbers

Monocular Numbers

You may have seen some numbers on them when scrutinizing it. When you turn your Monoculars away from you, you’ll see it has a model number.

The magnification is the Monoculars’ initial number, and it shows how close the images are together. The second digit indicates the objective lens diameter in millimeters after the x.

These are generally two digits separated by an “x,” corresponding to the Monoculars’ magnification and size. The magnification is indicated by the number before the “x,” while the lens size is displayed after the “x.”

So, if the Monoculars are labeled “842,” it indicates they have a magnifying power of 8 and a diameter of 42 (the diameter relates to the Monoculars’ objective lenses).

The accurate lens size is significant since it indicates the size of the Monoculars and hence the amount of light they will gather to improve your vision.

Dash Or Slash

You’ll notice a symbol between the first two digits, either a dash (as in our 12-4040 example) or a slash (as in our 10/3025 standard).

The dash denotes that the lens is zoomable. The slash indicates the presence of two distinct sets of eyepieces. A 10/3025 pair of Monoculars, for example, includes both a 10X and a 30X magnification lens.

Both designs have advantages and disadvantages. On the one hand, zoomable lenses allow you to swiftly zoom in and out, which is very useful for hunters since you can zoom in to view the fine detail and then quickly zoom out to track the game if it moves.

The zoom mechanism, on the other hand, increases weight and size. It’s also another moving element, which means it’s another item that might break.

If you’re carrying both sets of lenses, Monoculars with interchangeable lenses might be heavier and bulkier.

However, you may only bring your 10X lenses and leave the rest at home for a day of bird watching. This is also a useful feature for hikers, as space and weight are limited while carrying all of your belongings on your back.

Lens Diameter

After the x, the second number shows the objective lens diameter in millimeters, which influences the amount of light gathered and the extent of the field of vision.

This function is particularly useful in low-light situations, such as during sunset or twilight.

Monoculars with a larger lens diameter provide more brightness and visible details, but they also weigh more.

The heavier the Monoculars (or monocle) are, the bigger the lenses are. Monoculars with a diameter of less than 50 mm are usually recommended for travel, birdwatching, and outdoor sports since they are light and easy to carry.

A diameter of 20 mm is suitable for trekking and day observations, while a diameter of 50 mm ensures better visibility at night.

How Numbers Affect The Field of View

It’s natural to believe that you need a lot of magnification with your Monoculars, but this has a drawback. When you increase the magnification, the amount of area you can see at once decreases.

The field of vision, often known as the seeing area, is measured in degrees or meters. If your field of view is really limited, you’ll have to move the Monoculars around a lot to find what you’re searching for.

When it comes to the field of vision, the optimal field of view for your Monoculars is determined by the sort of activity you want to do with them.



You’ll need a broad field of view for hunting since you’ll need to keep an eye on moving targets. A magnification of around 8x would suffice.

When it comes to lens diameter, 42mm is a very common choice among hunters. This works well when visibility is low since it gives you a larger field of vision to monitor animals.



If you want to see vistas via your Monoculars while trekking, you’ll need a higher magnification – aim for 10x or 12x – so that you can focus on distant things.

The field of view will be reduced, which is OK. A diameter of 25mm or 28mm will provide clarity and brightness when it comes to lens diameter.

Bird Watching

Bird Watching

A magnification of 8x is sufficient for tracking moving things. Higher magnification is required if you wish to focus on motionless birds. A lens with a diameter of 40mm or 42mm will work well in low-light situations.

Sporting Events

Sporting Events

If you’re going to a sporting event with your Monoculars, aim for a magnification of 8x to 10x. This allows you to get closer to the action on the field or pitch while maintaining a wide range of vision. To view more details, the lens should be approximately 42mm.

How Numbers Affect Focus

You must also consider features such as the objective lens size (diameter) and magnification when focusing your Monoculars.

Because the Monoculars’ lens collects light, the larger it is, the more light it allows. As a result, a larger lens will ensure that the image you’re looking at is clearer and more detailed.

When it comes to magnification, the Monoculars have a lot of it. While magnifying an object can help you see it in greater detail, it can also make it difficult to see the image’s sharpness.

It will begin to become hazy. The Earth’s atmosphere, where heat waves and wind can cause various temperatures to blend, can also cause the view from your Monoculars to be blurry at high magnification.

This causes the air to interfere with the light you’re trying to capture from a distant object, and it usually becomes a problem once you reach 60x magnification. As a result, Monoculars’ higher power isn’t always the most important feature!

Objective Lens Size

The objective lens diameter, in millimeters, is indicated by the last number in your Monocular’s description.

The second number on my “8×42” indicates the diameter in millimeters of the main objective lenses.

The objective lens’s surface area collects light from the scene and transmits it to the ocular lens (then transfers it to your eye).

Because a larger objective lens captures more light, the image seen is likely brighter than one with a smaller aperture.

The more light that can be captured and transmitted to the eye in low light conditions, the better.

The surface area of a 42mm objective lens is four times that of a 21mm lens, allowing it to capture four times the amount of light (area = pi x radius squared).

Why do Objective Lens Matter?

You’re looking at a bit of a trade-off once more. The objective lens’s size determines how much light it lets in.

The brighter the image and the more vibrant the colors are, the more light it lets in. These large lenses will allow you to appreciate the plumage of birds better if you’re using your Monoculars for bird watching.

The objective lens, on the other hand, is the heaviest part of your Monoculars. The tubes can be made of aluminum, which is a lightweight material.

Plastic or rubber can be used for other factors. However, there is no substitute for high-quality optical glass, which is a fairly heavy material for lenses.

Generally, objective lenses with a focal length of less than 25mm (roughly an inch) will not provide a clear image at any significant zoom level.

With a magnification range of 25 to 40mm, you’ll have a good pair of Monoculars for hiking. A full-size lens with a focal length of 40mm or greater is ideal for bird watching, though it may be too large for hiking or backpacking.

Right Balance

If larger lenses lead to better quality, brighter images, why not just get the biggest lenses you can.

The downside to this is that it makes the instrument bigger and heavier and thus less convenient to carry about.

Where light gathering and low light performance are critical, like astronomy, for example, Monoculars can have very large 100mm lenses or even more in some cases.

However, this means that they are now so large that they have to be attached to a tripod to use them, which is certainly not ideal if you need easy to carry Monoculars for birdwatching when out in the field.

You can also have too much of a good thing with the Monoculars supplying your eyes with more light than they need.

This relates to the size of the pupils in your eyes and how wide they are dilated depending on the available light and concerns the length of the shaft of light exiting the eyepieces. 25×150 Monoculars capture over twelve times lighter than 10×42.

How Numbers Affect the Exit Pupil Size

The exit pupil size of your Monoculars is another important thing to consider when using Monoculars.

It refers to the width of the beams of light that are leaving the Monoculars’ eyepieces. It affects how well your pair of Monoculars will hold up in bad light conditions.

So, the larger the Monoculars’ exit pupil, the more light will be able to reach your eye.

You need to know the size of your pupils so that you can choose the best exit pupil size for your Monoculars.

This is because everyone’s eyes are different, and different people’s eyes will expand to different sizes according to the light outside.

If the exit pupil of your Monoculars is too large for your eyes, this can cause you to lose out on some of the light that enters the Monoculars.

How you calculate it is by dividing the objective lens diameter by the magnification of the Monoculars. So, if your Monoculars are 8×24, it means that the Monoculars have an exit pupil size of 3mm.

If you’re trying to view birds or wildlife, or doing other activities, with your Monoculars during daylight hours, then exit pupil size won’t matter much.

At night, however, you want an exit pupil size that’s at least 6mm as this will help you see more detail, such as when looking for stars in the night sky.

How To Calibrate Your Monoculars?

Start by turning the eyecups counter-clockwise so that they’re away from the body of the Monoculars. If you wear glasses, keep them retracted.

This allows you to have a wider field of view. If your Monoculars have a rubber cup that goes around the eyepiece, use it for comfortable viewing.

Follow the below steps to calibrate your monoculars-

  1. Hold the Monoculars by the barrels and look through them. Bend your Monoculars up and down at the center to ensure that both your eyes can fit over the lenses. When you look through the Monoculars, you should see a circular image.
  2. Adjust the barrels if you feel that the Monoculars are not fitting on your face properly.
  3. Next, to focus your Monoculars, you will have to look at an object in the distance that’s about 30 feet away from you.
  4. Cover one of your eyes with your hand. Is the image you see through the other eye blurry? If so, you need to adjust the focusing ring located in the center of your Monoculars.
  5. Then, close your other eye and focus on the image with the vision that’s uncovered. If you can’t see the object clearly, you have to use the diopter (wheel on the eyepiece) to adjust it.

Types of Lenses


They are appreciated for their impact resistance during extreme activities. The images produced have lower quality; consequently, the price is lower.


They are mostly used in professional monoculars and Monoculars. They produce quality images, and the price is higher. The glass reflects the light that enters from the outside, but this phenomenon is minimized through an anti-reflective treatment on the lens.


1.  What does first numbers on monocular means?

The first number represents the object’s size; it means how much larger that object you will see.

When a monocular has the first number as 12, it will show you the thing 12 times greater than the monocular’s original size.

If you want to see the larger objects, you should choose the larger number for complicated tasks such as stargazing or visiting the depth of water or laboratories.

But you can go with a smaller number if you need it for normal routine use.

Monoculars come in various ranges from 8x to 40 x, but commonly using monoculars are 10x and 12x.

But it depends on you which one you select according to the nature of the object you want to see with monocular.

2.  What does the second number on monocular means?

The second number on the monocular represents the diameter to show the image clearer and brighter.

Larger will be the diameter of the object, clear will be its view. The diameter number indicates that the diameter in millimeters.

For example, 50 means the diameter that the observing object’s monocular will show will be 50mm.

It would be best if you chose the monocular diameter according to the nature of the observing entity.

There are wide ranges of diameter in monoculars such as 40, 60, and many others.


You can use your Monoculars to see various sights – you can even use them for astronomy! In this article, we’ve provided you with information to help you use your Monoculars more effectively, whether you’re using them to birdwatch, spot wildlife, or locate a planet.

The diameter of the objective lens largely determines how much light your monocular can gather. The larger the objective lens diameter, the more light that the monocular will capture.

More light means a brighter view, particularly in low-light conditions. The downside is that the bigger the lens is, the heavier and bulkier your monocular will be.

Users who want to enjoy a wider, sharper, and brighter image often opt for lenses with an objective lens diameter of 30mm or over.

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