Breaking Down Binoculars: What You Need to Know and What Terms Really Matter

A variety of Different Binocular selections

With the accessibility of knowledge so readily available at our fingertips these days, it is often easy to find yourself in a rabbit hole of research when making purchase decisions and looking for new gear. There is so much information out there and so many products on the market that the process of making a purchase can often feel daunting and almost impossible. As a nature enthusiast, western hunter, birder, and all-around outdoors man, I find myself in a constant quest to find the next best piece of gear. I seem to always be looking for something that will make my life easier in the field, on hikes, and in my everyday outdoor pursuits. For me, one of the biggest purchasing decisions I have made (and have been making) over the past nine years has been trying to find and purchase the perfect pair of binoculars.

I’ll admit, the journey has not been easy and after years and years of online research, hard-earned money spent, and constant trial and error, I am still figuring it out. As such, this article has been written to help you make a confident and educated decision when purchasing your next pair of optics. In this article, we will break down the terminology and dive deep into the things that actually matter when purchasing optics.

First off, one of the most important things that you need to understand when purchasing a pair of optics is the terminology that is used by the manufacturer. Terms like: Diopter, Angular Field of View (AFOV), exit pupil, field of view (FOV), depth of field, Twilight factor, and eye relief. These are all terms that help break down a binocular. And yes, they sound really cool, but what do they actually mean? Better yet, how do these things help you in the field and how are they applied to a good piece of glass? Now before I dive deep into the terms, the first thing that I want you to consider when purchasing a quality pair of binoculars is your use for them.

Do you plan on using them in low light conditions such as early morning or late at night? Do you plan on only using them during the day? Or do you actually just want to have a pair to throw in your truck to use on occasion? Your use for a pair of binoculars is going to be a huge determining factor to consider when making your purchasing decision. Keep in mind that this article is not meant to sell you on a specific brand, but instead help you break down the terminology so that you can make an informed and educated decision when you start the buying process. So now that we have gone over all of that, let’s get in to the meaning behind the terminology.

the exit pupil in full view

In the world of optics, the exit pupil is an important term to understand. The exit pupil is a virtual aperture in an optical piece of glass. What is an aperture you say? Well in layman’s terms, an aperture is essentially a hole, gap or opening that lets light through. But more specifically, it is space through which light passes in an optical instrument. On a pair of binoculars, the exit pupil is seen as a bright circle in the center of each eyepiece. You can see this circle when you hold a pair of binoculars towards the light at arm’s length from your eyes.  Another way that you can see an exit people is by completing the following steps:

Fully retract the eye cups and focus the binoculars in a bright location.

Hold a white card up to the eyepiece of your objective lenses.

This will in turn project a disc of light onto the card that you are holding.

Once you see the disc of light, move the card closer to or further away from the eyepiece until you get the smallest possible disc diameter on the card.

This diameter is the actual diameter of the exit pupil.

Keep in mind that when viewing the exit pupil in a pair of binoculars, the disc should always be round and uniform in brightness. If shadows are visible, this could be an indication of poor workmanship.

So now that we have gone over what an exit pupil is and how to find it. The next thing that you are probably wondering is why is it important. Well, the exit pupil is important because it allows light to pass through your binoculars and into your eyes. So, in theory, a smaller exit pupil allows less light in and a larger exit pupil will deliver more light to your eyes. This can be an important factor to consider when comparing binoculars, especially when you are considering using them in poor light conditions such as dawn or dusk. But wait there is more, before you run to the store and buy a pair of binoculars with the largest exit pupil you need to consider a few other factors.

First off, the human eye varies in diameter based on the current viewing conditions. In normal daylight conditions the average human eye opens from 2 mm to 4 mm, and in dark and poor light conditions the maximum pupil size of a human eye is typically between 5 mm to 9 mm; depending on a person’s age.

To make it even more complicated, your pupil size will actually decrease in size slowly as you grow older, but on average, a person 25 years of age or younger usually has a pupil size of 7 mm in low light conditions.

So now that we understand that the human eye’s pupil size varies depending on conditions, let’s take a look at some practical examples to help us better understand how, why, and when these things matter. For example, if you take a pair of 8X56 binoculars you can calculate the exit pupil with a simple formula.

56/8 = 7.0 mm

So, based on this calculation, if you use a pair of 8x56 binoculars in bright conditions, and your eye pupil diameter is only 2 to 4 mms, then over half the light is being blocked by the retina in your eye. However, despite this loss of light, the emergent light still fills the pupil and subsequently there is no loss of light between your eye and the optic. Similarly, if you are using a pair of 10X25 binoculars the diameter of the exit pupil would be 2.5 mm. This means that the exit pupil in normal light conditions is about the same size as the normal pupil of your human eye. The result is an image that fills the human eye with light. So, in effect, the smaller binoculars would seem just as bright as a pair of 8x56 binoculars. This means that in normal daylight conditions you can get away with a smaller pair of binoculars because the human eye is essentially the same size as the exit pupil of a smaller pair of binoculars. We have now covered normal light conditions, let’s take a look at a similar scenario, but in low light conditions.

In low light conditions the 8x56 pair of binoculars will have a 7.0 mm exit pupil and the human eye will also have an exit pupil of around 5 mm to 7 mm. So, in this instance, the human eye is about the same size as the exit pupil of the binocular and therefore no loss of brightness is observed. The result is an image that is just as bright as if you were to see that image with your naked eye.

 In contrast, the smaller 10x25 binoculars will not let as much light in because the exit pupil of the optic is smaller than the 7 mm human pupil. This results in an image that is darker than what you would see with your human eye.  From these examples you can see that in normal light conditions you can get away with almost any size binocular, but in low light conditions you want a larger binocular with a larger exit pupil. Thus, the ideal exit pupil diameter that you need will depend on your specific need for that optic. In particular, if you are a hunter, or someone who likes getting up in the early morning or late evening to view wild life, you are probably going to want to look at an optic with a larger exit pupil. While on the other hand most compact binoculars are going to be plenty sufficient for daytime use. Wow that was a lot of information… but wait we still have more…next let’s take a look at depth of field.

The Depth of field of a binocular or spotting scope is the distance from “near to far” that is in focus at a certain setting of the focus adjustment wheel or at a certain distance.  Moreover, the depth of field decreases as the magnification increases, and at high magnifications the depth of field can be very shallow and precise focusing becomes critical. As such, the size, action, and feel of the focusing wheel or knob can be another important factor to consider when purchasing an optic especially when glassing free hand or without a tripod. Although depth of field is not the most important term to consider, it is still worth mentioning and something that you should consider when making a purchase. So now that we have covered depth of field let’s take a look at the term Field of View (FOV) and its importance in the world of optics.

In essence, Field of view (FOV) is the horizontal width of the image you can see while looking through a pair of binoculars at distance. In my opinion, field of view is one of the most important aspects for detecting critters in the early morning and late evenings. Having a large FOV allows you to pick up more movement on a hillside. This is due to the simple fact that you can see more out of the objective when looking through it.

 Thus, when buying a pair of optics, you will want to consider the field-of-view when making your selection. In particular you will want to look at each particular make and model, as each model is constructed differently and will thus have different FOV even though their magnification ratings are the same. Often when reviewing the manufactures specifications, the FOV will be represented by the number of feet per one thousand yards of distance. This term is known as linear field of view. You will also see in the manufactures specifications two other types of FOV and these are the Real field of view (RFOV), and the Angular Field-of-view (AFOV).

Real field of view or degrees field of view is measured from the center of the objective lens and expressed in degrees. When referring to RFOV, the lower the magnification the binoculars have, the wider the real field of view and the higher the magnification, the narrower the field of view. On the other hand, Angular or Apparent field of view (AFOV) is the angle at which your eye would move through the binocular, if you were to look at one edge and then scanned over to the opposite edge.  You can calculate the AFOV by taking the value of the RFOV and then multiplying that by the magnification.

For instance, if you had a pair of 10 power binoculars that have a 5 Degree field of view the apparent field of view would be 50 degrees. This value represents the field of view that you would see while looking through the binoculars. In general, and apparent field of view of 62 degrees or more is considered a wide field of view. So do you understand FOV now or does it at least make more sense? I hope so because we are now going to move on to the optical term Diopter.

The Diopter is a unit of measurement of the optical power of a lens. It is equal to the reciprocal of the focal length measured in meters (1/meters) and is therefore a unit of reciprocal length. Meaning a 4- diopter lens brings parallel rays of light to focus at ¼ meter. Basically, all this means is that the diopter measurement is used to help correct the image and help with clarity. The diopter is usually adjusted using a separate eyepiece-focusing tools called a Diopter Adjuster. The diopter adjuster is usually located on the right side of your binoculars and plays an important role in correcting the focus of your binoculars to your eyes. The term used to describe this adjustment is Dioptric correction, which is the adjustment of one lens to provide compatible focus when the viewers eyes have different capabilities. Adjusting this results in less strain on the eyes and allows for optimal viewing performance in the field. Ok so we have now covered a plethora of terms and we still have a few more to go…so saddle up and let’s get to it. The next terms that we are going to cover are directly related. They are eye cups and eye relief.

Eye cups essentially are used to help keep the distance from the ocular lens to one’s eyes and they can also help keep stray light away from one’s eyes while using binoculars. Almost all eye cups are rubberized and there are usually three types of eyecups in the world of optics: eye cups that roll, eye cups that slide, and twist up eye cups known as helicoid eyecups. The rubberized rolling eye cups tend to break from the constant rolling and are probably the cheapest option that you can find. The sliding eye cups are better quality but they can be hard to keep in place. Lastly, the twist up eye cups, are probably the best option because they can be left in any position from all the way down to all the way up. They are the best option to choose because they often have click stops at regular interludes. This allows you to get the best eye relief for your vision and can relieve strain on your eyes during long periods of glassing. Now that we have covered eye cups, lets dive into eye relief.

Note that if you do not wear glasses, you can skip the below section on eye relief; none of this will matter for you. Binoculars with long or short eye relief will work just fine for any person who does not wear glasses. If the binocular has long eye relief, the eye cups can be extended to hold the eyes the correct distance from the eyepiece. With that being said, if you do wear glasses you will want to read the section below on eye relief.

Eye relief is the distance between the ocular lens and the eye at which the eye can obtain the full viewing angle. This basically means that it is the distance at which an optical instrument can be held from the eye and the full field of view can still be observed. Generally speaking, the higher the magnification of the optic the larger the intended field of view, the shorter the eye relief will be.

In particular, eye relief is import for eye glass wearers because their eyes are typically further from the eye piece, and thus, a longer eye relief is often needed to see the entire field of view through a pair of binoculars. So, if you wear glasses, you will want to focus on binoculars with long eye relief, this will give your glasses the room that they need to get closer to the eye pieces of the binocular. In general, most glasses wearers need binoculars with a minimum of about 16 mm of eye relief. However, exactly how much eye relief one needs, depends on their glasses and their face structure.

For instance, if your glasses are small and they ride close to your eyes, you might get away with a binocular that has an eye relief of only 15 mm. However, most glasses wearers, need longer eye relief than that. To determine what eye relief you need, first try the binoculars with your glasses. Look through it with your glasses on and then with your glasses off. Do not worry if you can’t focus the binoculars without your glasses. What you are trying to determine is whether the image includes everything with your glasses on that you can see with your glasses off and that is how you will determine if the binoculars will have the correct eye relief for you.

Wow, if you have made it this far you will be happy to know that we are almost finished. The next term we will cover in this article is focal length, so here goes nothing.

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which paralleled rays are brought to a focus. A system with a shorter focal length has greater optical power (not to be confused with magnification) than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance. In general optics with a shorter focal length are better for viewing larger distances. Shorter focal lengths not only capture a wider view, but they also have a deeper depth of field. They can capture almost everything from right in front of the lens to infinity in acceptable focus. So, when you are making a purchase decision, definitely take focal length into consideration.  The last term that I want to cover in this article is twilight factor.

In essence the twilight factor is a formula used to determine a binoculars performance in twilight. A binoculars twilight factor was more important in the past, before modern optical coatings were invented, but even so, it is still worth mentioning. In a modern binocular, the performance in poor light now depends more on the quality of the glass and optical coatings than on just the twilight factor formula. Good coatings can double the amount of light that moves through a binocular when compared to those that have poor quality coatings. Twilight factor is the most useful rating for low light performance. The higher the twilight factor is, the smaller details we can see at dusk.  The twilight factor alone does not permit any kind of real statement. A correspondingly large exit pupil is always decisive and important for twilight use. Ideally, it should be at least as large as the pupil of the user. Anything with an exit pupil smaller than 4 mm is usually not suitable for viewing in low light conditions. And with that we have come to the end of our optical journey.

Simply put, you can’t pick a quality pair of binoculars out if you do not understand what makes a binocular quality. Often times the terminology and definitions used by optical manufactures can be vague and hard to understand. To better understand what you are looking for in a quality piece of glass you must first understand the meanings behind the terms and specifications associated with those products. To gain a better understanding of this, we described and looked at some of the most import terms and definitions to consider when purchasing a quality pair of binoculars. Ultimately, Knowledge is always power, and you now have the power to make well-informed decision. Hopefully, this article will prevent you from making the same rooky mistakes that I made in the past when purchasing glass.

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Written by Christopher Bunce 9/14/2023