Does Binocular Lens Size Make a Difference

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Does Binocular Lens Size Make a Difference

The lens size of a pair of binoculars is very important and is one of the first aspects a potential buyer should consider when browsing the different makes and models available on the UK market. Fortunately, binocular specifications make it easy to see at a glance how big the lens diameter is. Binoculars are classified by both magnification and lens size, so a pair of binoculars listed as being 7x35 is capable of magnifying an object to seven times its original size and has a lens diameter of 35mm.

Why Binocular Size is Important

The lens diameter of a pair of binoculars is vitally important for three reasons. Firstly, the lens diameter determines the interpupillary distance, or exit pupil, which should be matched the individual user. Secondly, the size of the lens affects how much light travels through the lens. Naturally, a larger lens will let more light through than a smaller lens. The more light that can travel through the lens, the brighter the viewed image will be. This means that if the binoculars are intended to be used in darkened conditions, such as for stargazing, or in limited lighting conditions, such as at the theatre, a larger lens will produce a better image. A brighter, clearer, sharper image also means a higher resolution, so intricate detailing can be viewed much easier than with a smaller lens.

Choosing the Right Binocular Size

There are both advantages and disadvantages to small and large binocular lens sizes, and the lens size that is chosen should depend on the specific features of both and the intended purpose of the binoculars in general. Generally, speaking, try to look for a lens size that is no less than five times the magnification.

Small Lens Features

Binoculars with small lenses are typically not as high quality as those with larger lenses, but they do come with their own advantages that make them the preferred option for many, particularly those who value portability.

Size of Lens

Small lens sizes range from around 20mm to 30mm in diameter, much more compact than larger lens sizes which can reach up to 50mm in diameter.


Binoculars with small lenses typically have lower magnification levels than larger lenses. Expect these models to be able to zoom to less than 8x , up to a maximum of 8x to 9x .

Field of View

Generally, small lens binoculars will have a very wide field of view, due to the lower magnification. This means that a wide landscape can be viewed, rather than a single, small object.

Exit Pupil

The smaller the lens size, the smaller the exit pupil, or interpupillary distance. For optimal results, exit pupil should be matched to the user, so children may be particularly suited.

Binocular Size

Small lenses means far less body is needed to hold them in place, so small lens models are often very small and portable, with many types of compact binoculars for convenience.


Not much light can travel through small lenses, so the image that is viewed through these binoculars will generally be lower quality, with lower resolution. They are best used in good light.

Large Lens Features

Binoculars with large lenses allow much more light to travel through, providing a higher quality image with excellent resolution. However, they are not suited to every user and are often much more bulky in appearance than the smaller, compact models.

Size of Lens

The lens diameter of these larger versions ranges from as low as 30mm to as high as 50mm. The larger the lens, the more light that will be let through, creating more advantages.


Unlike smaller lenses which typically have a low magnification, large lenses are available with both low and high zoom levels. The higher the zoom, the more detail is available to the viewer.

Field of View

The field of view varies depending upon the level of magnification, so both wide and narrow field of view is available with large lenses. A narrow field provides more detail.

Exit Pupil

Large lens binoculars have a larger exit pupil, or interpupillary distance, due to the greater diameter. For optimal results, these binoculars are best used by those with a naturally large exit pupil.

Binocular Size

Binoculars with large lenses are often much bigger in size and in weight than the smaller models. This makes them less portable, which is something walkers and hikers should consider.


As a large amount of light can travel through the large lenses of the binoculars, the image viewed is very bright with a high resolution, making them good for use in limited lighting.

Intended Purpose

It is important to consider the intended purpose of the binoculars before committing to a specific lens size. Due to the differing features of small and large lenses, different models are particularly suited to certain activities and events.

Sporting Events

For events in a large stadium, the lens size really depends on personal preference. Good lighting conditions mean both small and large lenses would be adequate for this activity.

Water Sports

For use out on the water, look for binoculars with a large lens diameter. Do not forget to purchase waterproof binoculars, preferably with nitrogen-filled rubber for protection.


Due to the low lighting conditions of a theatre, look for binoculars with a large lens diameter of at least 30mm. As the focus is the small stage, a high magnification would be good.

Walking and Hiking

If walking in the open, small lenses with a low magnification and wide field of view would be adequate, but if hiking through covered forests or woods, choose a large lens to let light in.


As with walking and hiking, if hunting in open fields, a small lens diameter would suffice, but if hunting in forested or wooded areas, a large lens would be a better option.


Large lenses of at least 30mm, but preferably 42mm to 50mm are recommended for birdwatching. Also choose a high magnification for sufficient zooming ability.


When used in darkened conditions, binoculars really need a very high lens diameter to work optimally. For stargazing, consider a 50mm lens with low to mid magnification.

Understanding Interpupillary Distance

The lens diameter determines the interpupillary distance, or exit pupil, of the binoculars. This is the distance between the two points of the lens that the image is viewed through. This means that users with a naturally large interpupillary distance would see better through binoculars with a large exit pupil, as the eye and the lens would line up, rather than being off-centre. Lighting conditions will affect interpupillary distance, so it is important to take this into account.

In dark conditions, the pupils open wider to between 5mm and 7mm, so a larger exit pupil, and larger lenses, would be preferable. In bright conditions, interpupillary distance may be smaller, fitting better with smaller lenses. When buying binoculars for children, small lenses with a small exit pupil often fit children's eyes better than large lenses. Individual interpupillary distance can be determined by a trip to the opticians.

Find Binoculars with the Right Lens Size on eBay

Binoculars with both small and large lenses can be found for sale on eBay and are simple to locate, browse, and purchase. eBay's Electronics store is the place to start. From here, select Cameras & Photography, followed byTelescopes & Binoculars, and finally, select the Binoculars & Monoculars category. Use the left hand menu to filter the results to see only those items listed as binoculars, and browse listings to find a pair of binoculars that meets all needs. Remember to look at the specifications, paying particular attention to the numbers, such as 7x21, which refers firstly to the magnification, and secondly to the lens diameter. If the results seem a little overwhelming, be sure to narrow them further by utilising the left hand menu. This allows potential buyers to see only products with a specific magnification or a certain lens coating.


Does binocular lens size make a difference? Absolutely. The diameter of the lenses is one of the first aspects that should be taken into account when searching for a new pair of binoculars. There are advantages and disadvantages to both small and large lenses, so it is impossible to say which is better. Instead, potential buyers should consider what they intend to use the binoculars for before settling on a specific lens size. In a nutshell, if planning to use the binoculars in the daytime in adequate lighting conditions, a small lens size should be fine unless the user has a particularly large interpupillary distance. On the other hand, if planning to use binoculars at night or in conditions with limited lighting, opt for a larger lens that lets more light in. If in doubt, follow the general guideline of choosing a pair that have a lens size five times greater than the magnification level.

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