We have covered binoculars on the site before. We’ve explained about about how they work and also recommended some of the best binoculars you can buy. But you may be wondering what goes into picking those binoculars. Maybe you don’t need a super expensive pair. Maybe you need a compact pair. Or ones that offer night vision capabilities. Or maybe a cheap and basic pair won’t meet your needs either.

If you want to know what an objective lens is, or a porro prism, this article is for you. We’ll cover everything from eye relief to light transmission. From field of view to what fully multi coated lenses are. Let’s get into everything you need to know when it comes to buying binoculars.

What Do You Need Binoculars For?

This may seem like a simple question at first. You need binoculars to see things close up, right? But there’s more to it than that. Something like hunting requires different binoculars than everyday use. If you want to sit in a chair and see a race more clearly, a lower magnification is better. That’s because it will open up your field of view more. This can be good for bird watching, or concerts, or just random fun.

If you’re hunting, you need that maxed out magnification. Your field of view will decrease as a result. But you’ll see at a greater distance with more clarity. That’s key for a hunter.

If you want to do something like bird watching, things change again. A larger lens size is great for bird watching. This gives you a wider field of view so you don’t miss anything. The narrower the field of view, the harder it is to pinpoint and focus on what you want to see.

Obviously, this is all a balancing act. No one wants just one feature from their binoculars. An ideal pair has great magnification and field of vision. But achieving both in one pair can be difficult. And the way they’re made means one feature tends to be sacrificed for the other.

So how do you even know if a pair offers good magnification? And what makes a better field of vision? Let’s break it down.

Magnification vs Objective Lens Diameter

All other features aside, this is the one that causes the most confusion. If you’re new to binoculars, you’ll see that they are all categorized by numbers. If you have to choose between two pairs, one is 8X42 and one is 10X25, which is better?

The two numbers refer to magnification and objective lens diameter. The first number is the magnification power. The second number is the objective lens diameter in millimeters. The objective lenses are the large lenses at the end of a pair of binoculars.

The larger the objective lens number, the bigger overall the binoculars will be. If you don’t even see a picture of the binoculars themselves, the numbers will tell you a lot. In our example above, you know that the 8X42 pair is much larger than the 10X25. The lenses are almost twice the diameter.

Getting a feel for lens diameter size is important. Big binoculars are, for most people, harder to manage. They are heavier and clunkier the bigger they get. So if you just want a tiny pair around your neck for birdwatching, you may not want the biggest lens diameter out there.

Higher magnification binoculars always sound good. But remember, that comes at a cost. More magnification generally leads to less stability. The further away something is, the shakier the image becomes when you’re viewing it. This is often down to a simple matter of physics. If you are focused on a small object at a distance, holding it in view is harder to do. If your hand shakes even a tiny amount, you can end up losing your target. You’ve probably experienced this before.

You may have heard that military snipers have to time shots between heartbeats. This is a great example of how this issue with binoculars works. The scope on a hunting rifle or, in this case, a sniper rifle works like binoculars. If you are focused on something very far away, then maintaining a line of sight gets harder. At extreme magnification, your own heartbeat can vibrate the scope. So much so that it rises or falls away from your target. It’s so far away they need to time shots between heartbeats. That way, the tiny vibration doesn’t alter the path of the bullet when it’s fired.

Image sharpness or clarity is also affected by magnification. What you see with a lower magnification pair of binoculars will appear as a sharper image. For higher magnification, your focus will need to be adjusted to try to make it sharper.

Brightness is also a factor here. The kind of lenses and prisms used in binoculars affect image brightness. Some are able to strip away certain colors of light. Depending on what you need and what you expect, this can be good or bad. It’s always best to try binoculars firsthand before buying whenever possible. If you can’t, then definitely read reviews and check out test charts or other features. Things that let you see and understand how the binoculars will show you images.

Speaking of how images are formed, let’s take a look at prisms. These are what allows binoculars to work at all. Light enters the lens of your binoculars and is flipped. This is because a convex lens refracts light. So right away the image is broken as the rays cross over. What you are looking at will now appear upside down. To fix this, the light hits a prism. That prism then corrects the image. It rights the image so you can see it properly. One prism will rotate the image 90 degrees. A second prism does another 90 degrees. That makes a full 180 degree rotation, taking the image back to where it started.

Roof Prism Binoculars

Roof prism binoculars are the smaller kind of binoculars. Usually, anyway. You can still get some large roof prism binoculars but they allow for smaller sizing. You can always find roof prisms that are smaller than porro prisms, even with the same stats. The reason for this is how they work.

At a glance, you can tell the difference between roof prism binoculars and porro prism ones. Roof prism binoculars are straight. The eye piece and the objective lens are perfectly in line. The prisms inside are arranged in a straight line. The light refracts around the first prism. It is then reorganized in the second prism. So even if it seems literally like a straight line, it isn’t.

Roof prisms offer a longer focal path. That means you can expect greater magnification. Because of how the prisms are arranged, they are also more durable than porro prisms. That said, roof prisms are more expensive as well.

So, roof prisms can potentially offer better magnification in a smaller package. They can stand up to more abuse, but you may pay more as a result. Cheaper roof prisms may sacrifice image quality. Keep that in mind.

Porro Prism Binoculars

This is the older prism technology. But that doesn’t mean it’s less effective. You can tell porro prism binoculars at a glance because they are offset. The objective lenses are not lined up with the eye pieces. That’s because the image comes in through the objective lens and hits the first prism. It’s arranged horizontally, next to a second prism at 90 degrees. The image is sent from the first prism into the second. The image from that is then sent to the eyepiece.

Because the lenses are arranged side by side, they need more space. That means porro prism binoculars will always be bulkier. Despite the size difference, porro prisms are easier to make. The technology is actually many hundreds of years old. For that reason, these binoculars are usually cheaper than roof prism ones. The image quality is typically very clear.

A downside to porro prism binoculars is that they tend to be less durable. Because of how the prisms are set, they can become misaligned if you drop the binoculars. A prism can even be knocked right out. If that happens, the binoculars no longer work. Also, the designs for these kinds of binoculars tend to not be waterproof.

Prism Glass Grades

So we know the two types of prisms in binoculars. What does Bak-4 and BK7 mean? These complicate matters for some binocular users. Bak-4 and BK7 refer to the type of glass used in the prism.

Bak-4 is a higher density glass. It’s considered better quality. It also costs more than BK7. It has superior anti-reflective properties and can make for a clearer image. If you’re a serious hunter or bird watcher, you may prefer Bak-4.

BK7 is a lower density glass. The difference in images it produces is almost negligible for most users in daylight. Like we said, if you’re serious about optics, Bak-4 is better. But for casual use, BK7 is probably all you need, and it’s cheaper.

In low light or at night, Bak-4 is far superior. BK7 suffers some aberrations and will be harder to use. So again, it depends on what you want your binoculars for.

What Are Objective Lenses?

Let’s head back to the front of the binoculars. Those objective lenses have a lot of work to do. The objective lenses are the ones closest to what you’re viewing. These do not actually magnify anything. Neither do the prisms, for that matter. The objective lenses allow light into the binoculars. The larger they are, the more light they let in. That, in turn, means the more you can see. So if you have 8X42 and 10X25 binoculars, the first pair lets in more light. The magnification may be lower, but you will see more in the field of view. Likewise, the images with be brighter.

The Eyepiece

This is also called the ocular. In many binoculars, this is in the part called the eye cup. It’s where you place your eyes to see through the binoculars. Because these lenses are so small, some people don’t think they are as important as the objective lenses. In truth, both are necessary for proper functioning. The ocular is what receives the images from the prisms. It magnifies them and allows you to see the close up detail you’re looking for. So the entire binocular up until the eyepiece didn’t magnify anything yet. It all happens literally right before your eyes.

The eye piece also determines, in part, the field of view. It works in conjunction with the objective lenses on that part. It’s also what causes the edge of image clarity to be strong or weak. Some binoculars show a good image in the middle, but the edges will be blurry or fuzzy. A higher quality eye piece tends to fix this.

What is Eye Relief?

The distance between your eye and the eyepiece that allows you to see the whole field of view. If you wear glasses, this is an important factor in choosing binoculars. Most of us who wear glasses would rather not take them on and off to use binoculars. With the right eye relief, you won’t need to. But if there’s a small eye relief, then your eyes need to get closer to the eyepiece. And that means your glasses will be in the way.

Eye relief can often be adjusted. Rubber eye cups and other features allow users with and without glasses to use them.

What is the Exit Pupil?

Point a pair of binoculars at a window in the daytime and look at the eyepiece. There will be a little circle of light visible there. That’s your exit pupil. It’s the tiny hole in the ocular that allows your eye to see images. A higher number exit pupil means more light getting to your eye.

The exit pupil diameter is not one given with most binoculars. Not like that 10X25 number we mentioned. But it’s also very easy to calculate. Divide the objective lens diameter by the magnification to get your exit pupil. In this case it’s 2.5. So that means you can expect a 2.5mm shaft of light to hit your eye with this pair.

During the daytime, exit pupil diameter isn’t super important. But as the light begins to fail, it has a noticeable impact. If you’re using binoculars at dusk or dawn with weak light, a higher number is always better. If you’re a hunter or a bird watcher active at those times, look for 5mm or more. Something like a pair of 8X42 binoculars would be ideal. But if you don’t plan to be using the binoculars in low light, don’t worry about it that much.

Field of View

The field of view allows you to see more. It’s a simple factor to keep in mind. If you want to go bird watching, you probably want a wider field of view. A narrower field with better magnification is great if you already found the bird you’re looking for. But since bird watching involves a bit of hunting, a wider field of view is usually better.

As a general rule of thumb, the lower the first number, the better the field of view. But that also means the lower the magnification. So you need to find that perfect spot that meets your needs. If you’re birding, 8X42 or something in that ballpark is probably greater. Big lens diameter to let in lots of light, and a decent field of view. If you’re hunting, bump that first number up. 10X42 would be a good number to go with. If you are doing long range hunting or skygazing, then there are specialized kinds. These can get up to 25X100.

How to Choose Binoculars: Other Factors to Consider

The lens and prism types are really just the basics of choosing binoculars. If that’s all you know about a pair, you’ll probably be OK. if you just want something to enhance hiking, or even boating, maybe you’re good already. But there’s definitely more to know. And considering binoculars can cost hundreds and even thousands of dollars, it’s good to know why.

Lens Material: Lenses for binoculars are usually glass. But not always. You can get plastic lenses as well. Plastic is cheaper and doesn’t offer the same quality optics. Why use it at all? Durability. Plastic lenses can handle more abuse. If you have kids that want to play with some binoculars, you may want to start them on plastic.

Coatings: You’ll see things like “fully multi-coated” in descriptions of binoculars. This means the lens coatings, and there are different kinds.

In general, lens coatings prevent loss of light. Coated lenses transmit images better. More clarity, more brightness, and sharper overall. Around 5% of the light can be lost passing through a lens that isn’t coated. And your binoculars have several lenses inside. Cheap binoculars can lose over 30% of light before the images reach your eye. But with multi coatings that drops to less than 5%.

For casual users, just look to see if the lenses have been coated at all. Fully multi coated means all lenses have several chemical coatings. That will provide the best optical experience. But the wording is key. It has to say fully multi-coated.

If it just says “coated,” that means there is one anti-reflective coating and on at least one lens.

If it says “fully coated” then that means the coating is on the objective lens, the ocular lens, and the prism.

“Multi-coated” means multiple layers on at least one lens.

“Fully multi-coated” means every surface. That means the objective lens, the prism, and the ocular lens have multiple coatings. This is usually a sign of the highest quality binoculars. That said, it doesn’t always mean the best images. But it’s a factor to consider.

Risk Factors for Binoculars

Like everything in life, binoculars have risk factors. Things that you want to avoid whenever you can.

Weather: A good pair of binoculars will be waterproof and weatherproof. Those are two different things, by the way. It’s not just rain, but things like condensation you need to worry about. On a wet day or a humid day, binoculars can fog up. If you get moisture inside a pair, you will know true frustration. Once fog forms on the inside of the lenses, it’s all but impossible to fix without taking them apart. And taking them apart ensures more moisture can get in.

Some binoculars are designed for life at sea. These marine binoculars are so thoroughly sealed you can literally drop them in the ocean off of your fishing boat. These are ideal if you know you’re going to be working in wet conditions. The tubes are sealed with O-rings and filled with gas. That creates pressure and does not allow any moisture to enter at all.

You will see both weatherproof and weather-resistant binoculars. Weather resistant means they can handle a bit of light rain. But if they get saturated, they may be ruined.

Drops: No one ever wants to drop anything expensive. Whether it’s your phone, a priceless vase, or binoculars. That’s how you break stuff. Binoculars are no different. But what is different is how binoculars can break. Obviously the lenses are at risk of shock damage. But the prisms can be damaged as well. And it’s harder to notice a prism is damaged at first. With porro prism binoculars, it’s also easier to do. For that reason, you may want to choose more durable binoculars if you’re letting kids use them. Or if you know you’re going to be rock climbing or roughing it.

Binoculars designed for outdoor uses often have durable coatings. A rubber coating can improve your grip on the binoculars. It can also act as a shock absorber if they get dropped.

Size: Like we said earlier, sizes can vary a lot. Big objective lenses make big binoculars. Most bird watchers want a small pair that can hang around their neck. So you need to factor that into choosing binoculars. Check the weight before you commit. If you plan on holding these up for a long time, extra weight will fatigue your arms, eventually.

The Bottom Line

There are a lot of really great quality binoculars out there. Choosing the right one depends on a number of factors. As we’ve shown, you need to consider if you want higher magnification or a greater field of view. You’ll want to take durability into consideration as well. And another big factor, maybe the biggest, is price. Are you going to be using them enough to justify a few hundred dollars? Or is a pair under $100 all you need? Now that you know what goes into making binoculars and how you can use them, the choice will be a lot easier.