What Do Binocular Specs Mean

What Do Binocular Specs Mean?

A decent pair of binoculars will give you years of great service and pleasure. You’ll wonder at some of the details you’ll have access to that you never knew existed. Like the subtle patterns on some birds feathers, or how many stars you can see from your garden. In fact the visual world a good pair of binoculars opens up to you has to be seen to be believed.

But, as with all things, not all binoculars are made equal. There are so many brands offering so many features at such a wide range of prices and it can seem quite confusing and overwhelming to know which pair to choose. Read on and don’t worry, because we have all you need to know to make the perfect choice.

Be sure to read all of this article because you will find this to be the most comprehensive, straight-forward all you need to know about binoculars guide. We’ll keep it as short and sweet as we can. Let’s face it, we all have busy lives and don’t want to spend hours reading boring facts and figures that have no relevance on the terminology and necessary information about binoculars.

Which Type Of Binoculars?

This will be the first decision you’ll need to make, and you’ll probably already recognise the 2 main shapes of binoculars but not necessarily know their names. The two main types of binoculars are;

Porro Prism Binoculars

Porro prism binoculars are named after their inventor Ignazzio Porro and the same basic design has been around for nearly 200 years. Shaped like a capital M these classic binoculars start out narrow at the eyepiece and widen to a central bulk (which is where the prisms are set) then get even wider towards the large lens at the base. There are two prisms per lens tube and they’re offset to allow light to pass through and magnify the image seen through the lens. 

The design was brilliant for its day, but there are a few flaws that can cause unnecessary problems in the 21st century. Porro prism faults include;

  • Large
  • Heavy
  • Bulky
  • Almost Impossible To Fully Waterproof
  • Easy To Misalign The Prisms (due to the internal prism set up)

If the prisms do get knocked even slightly out of alignment, the first you’ll know of it will probably be eye strain and headaches. With that said, they have good points too, which include;

  • A Wide Field Of View
  • A High Light Transmission Rate
  • Better Depth Perception (compared to roof prisms)
  • Far Cheaper To Produce Than Comparable Quality Roof Prisms
  • Better Stereoscopic Vision

Roof Prism Binoculars

These were the product of some of the finest optical minds of the 20th century. Carl Zeiss patented the roof prism binoculars in 1905 and they have been developed and improved on ever since. Shaped like a capital H, they consist of two lens tubes separated and joined by a central focussing wheel. They take their name from the design of having the prisms set into the roof of the lens tubes.

Due to the design they need a tighter alignment of optical elements and a special “phase coating” that keeps the image clear and bright. All this extra precision work increases production costs, but once they are set, they’re set for life and will never need collimation (unlike Porro prisms that can get damaged with just the slightest of knocks).

There are a number of advantages to owning roof prisms compared to Porro prisms which include;

  • A strong, robust design
  • A small, compact size and shape
  • Fully waterproof And Fog Proof
  • Better Magnification
  • Lightweight

Roof prisms do have a few disadvantages too which include;

  • Narrower Field Of View (due to objective lens position)
  • 12 to 15% Less Brightness Of Image
  • More Expensive Production Costs
  • That Expense Passed On To The Customer

In an effort to compete with the compact design of roof prism binoculars, some manufacturers have developed a different type of Porro prism binocular.

Reverse Porro Prism Binoculars

Although these are smaller than regular Porro prisms, they are still large for compact binoculars, but they are relatively easy to waterproof and fog proof. They are also easy to pack/carry and weigh less than standard Porro prism binoculars. However, the objective lens is too close together which means the stereoscopic image is reduced, which also affects depth perception.

Which Size Binoculars Are Best?

That’s the main types of binoculars covered, let’s have a brief look at the sizes of binoculars. Binocular sizes come in three classes which are determined by the size of the objective lens diameter, and run as follows;

  1. Compact
    As the name suggests, these are the smallest sized binoculars with an objective lens diameter smaller than 30mm. Compact binoculars are handy for backpacking/hiking or any activity where size and weight are important.
  2. Mid Size
    These are the intermediate sized binoculars and have an objective lens diameter of between 30 to 40mm. Mid size binoculars are the most common binoculars for long term usage, due to their size and weight, they can be carried/held for long periods of time without causing any problems.
  3. Full Size
    Full size binoculars are the largest of the lot and have an objective lens diameter of greater than 40mm. Full size binoculars are used for more specific activities like stargazing, static bird watching etc. If they are very large or heavy or have a magnification greater than 10x they will need support on a tripod or similar.

Just because a pair of binoculars has a full size classification doesn’t mean they will be overly large, just that they have an objective lens diameter greater than 40mm. With that said, the larger the objective lens diameter the heavier the binoculars will be.

What Do The Numbers Mean?

Somewhere on the body of the binoculars you will find a set of numbers separated by the letter X. These numbers give us some important information about the size, power and image brightness of the binoculars. The first set including the X indicates the level of magnification and the second set indicates the size of the objective lens diameter in millimeters. For instance 8×32 tells us that that particular pair of binoculars has a magnification level of 8 times (which means everything viewed through the lens will appear 8 times larger than with the naked eye) and has an objective lens diameter of 32mm. 

More Is Less

When it comes to magnification, most people want as high as possible but that can be counter productive. This is because of the way the body is made, holding any object for too long causes our arms to shake. That shake is usually not detected at all or only slightly, but under high magnification, it becomes all too apparent.

Which is why we recommend a magnification of no more than 10x for hand held binoculars. Any binoculars above 10x should be supported via a tripod to maintain a clear image. For most people and hobbies 8x magnification is more than enough, but for specific uses check out our guide for general use binoculars for more details.

Objective Lens Diameter

One of, if not the most important specification of any pair of binoculars, why? Because the size of the objective lens diameter not only determines the overall size of the binoculars, but it also determines how bright the image seen through the lenses is. This is because the only way light can enter the lens tubes is through the objective lens. So the larger the objective lens, the brighter and clearer the image will appear to be.

The Exit Pupil

That beam of light that enters through the objective lens is called in binocular terminology the exit pupil. It’s an important feature because the exit pupil needs to closely follow the size of our own pupil because that’s how we can see objects. So in bright sunlight the human pupil shrinks to around 2 to 2.5mm and as light conditions diminish our pupils dilate to gather as much light as possible, reaching an average of between 5 to 7mm.

This means if you’re going to be using your binoculars mainly in good daylight conditions the exit pupil is not such a major concern. But if you intend to use your binoculars in low light conditions like dawn, dusk or on dull days, you’ll need a pair with a larger exit pupil. As long as you have the magnification and objective lens values, you can determine the exit pupil size.

Simply divide the objective lens diameter by the magnification the result will be the exit pupil in millimeters. Our 8×32 binoculars from earlier have an exit pupil of 4 because 32÷8=4, these will be perfect for daytime usage but for dull or darker conditions you’d need a larger exit pupil size.

The Field Of View

This is often stamped onto the body of the binoculars along with the magnification and objective lens diameter. It usually looks something like FoV 140m@1000m, this means that at a distance of 1000 metres you can see 140 metres across from left to right. Anywhere between 105m to 140m@1000m is considered good for binoculars.

Eye Relief

The eye relief is the perfect distance between the ocular lens (the lens closest to your eye) and your eye where you can see the full image through the binoculars with no blurry edges or black rings. Most binoculars have an eye relief of between 11 to 16mm.

Long Eye Relief

Long eye relief is only necessary if you need to wear glasses whilst using binoculars. It allows enough distance between the ocular lens and your eyes whilst accounting for the width of your glasses. You’d have to check the specs but long eye relief is usually between 16 to 24mm.


Interpupillary Distance (IPD) is the distance between the centres of both pupils. Everybody’s IPD is unique which is why binoculars have a hinge allowing you to adjust the distance of the lens tubes to accommodate your particular IPD. If your binoculars are not adjusted to your particular IPD you will not see the full image, and your image will be framed with a dark halo.


There are two types of focussing when it comes to binoculars, the most common is the central focus. This is where one lens has a dioptre which adjusts the lens on one eyepiece (usually the right eye) to counter any near or far sightedness, and a centre focussing wheel for general focussing. The other less common type is with a dioptre on each eye and no central focussing wheel. This second type is best for individual binoculars (where no one else uses them) but is limited to one particular focus point.

Light Transmission

As we said earlier, light is a very important factor so the more light transmission the better the image you can see through the lens of the binoculars. To improve light transmission, binocular manufacturers use high precision optical glass for the prisms and the lenses. The very best optical glass, used in the production of top quality prisms for the best binoculars is BAK-4 glass. This is highly polished to remove nearly all imperfections which allows greater light transmission.

The more common binoculars use BK-7 glass which is still high precision top quality glass but with slightly more imperfections.

Lens Glass

The actual lens also plays a large part in light transmission, the top manufacturers use Extra-low Dispersion (ED) glass for making their binoculars lenses. This ED glass gives a clear, crisp, sharp image with no colour fringing or haloing.

Lens Coatings

The coatings applied to the lenses of binoculars also plays a part in light transmission. There are 4 levels of lens coatings available ranging from;

  1. Coated – This means a single layer on at least one lens
  2. Fully-Coated – This means a single layer on all lenses
  3. Multi-Coated – This means multiple layers on at least one lens
  4. Fully Multi-Coated – This means multiple layers on all lenses inside and out.

Opt for Fully multi-coated (FMC) if you can as this gives the best light transmission and reduces glare too.

Protective Coating

Top manufacturers protect their binoculars with either a rubber or polycarbonate coating. This prevents any accidental damage from dropping or just general scrapes and bumps.

Waterproofing & Fog Proofing

Even if you only plan on using your binoculars in your own garden, there’s a good chance that at some point or another they’ll get rained on. Once water enters the binoculars, they’ll never be the same again. To be fully covered from any level of rain you need them to be waterproofed to at least the level of IPX6. This is an industry standardised coding system, to find out more about waterproofing codes follow this link

Fog Proof

Going from one temperature extreme to another can cause your binoculars to fog up. To counteract this the manufacturers purge all of the air from the lens tubes and replace it with an inert gas (usually nitrogen or argon). As the gas contains no moisture it cannot react to fluctuating temperatures and so cannot fog up.

Dust Proof

Once the gas is sealed inside the tubes it cannot escape, and nothing can enter either. This means the binocular lenses will be dust and mould free too. 


Not necessarily part of the specs, but an important feature well worth considering, the warranty should cover your purchase for a good length of time. Most of the top brands offer limited lifetime warranties on their binoculars. Which means as long as the original purchaser owns them they will be repaired or replaced in the event of a genuine fault.

Frequently Asked Questions

Is 10×50 better than 20×50?

10×50 binoculars have an exit pupil of 5mm, a large objective lens diameter to allow a good amount of light to enter and a 10 times magnification. 20×50 binoculars have a 2.5mm exit pupil which is only half as bright as the 10×50. As the magnification is so high they will be hard to use without a tripod.

Which is better 10×50 binoculars vs 10×42 binoculars?

From a light gathering perspective, the 10×50 are better but that doesn’t make them a better pair of binoculars, there are many other factors to be considered.