When I was new to the world of binoculars, I was mystified by the meaning of the number pairings that describe binoculars and are engraved on the binocular exterior. I didn’t know that the meaning of these numbers was fairly simple.
The numbers on binoculars refer to the binocular magnification and lens diameter. The first number refers to the binoculars’ magnification. The second number describes the objective lens diameter in mm. Therefore, an 8×42 binocular has a magnification of 8 and a lens diameter of 42mm.
I remember thinking the bigger the numbers, perhaps the better the binocular. I also thought that the bigger the physical dimensions of the binocular were, the higher the magnification must be. I was wrong on both counts.
This article uses simple language to describe the positive and negative effects of different magnifications and lens diameters on things like image quality, brightness, and ease of use, and how users can decide which type of binocular is best according to their needs.
If you are an eyeglass wearer who is searching for an excellent and affordable pair of binoculars, be sure to use this checklist.
What Do Binocular Numbers Mean?
The Numbers on Binoculars Refer to Magnification and Lens Diameter
Understanding binocular numbers is easy. Again, the number pairings – like 8×42 – refer to the binocular magnification and binocular lens diameter in mm, respectively.
To explain binocular magnification (like 8x or 10x) in practical terms, magnification means how many times closer an object appears to us when viewed with binoculars compared to when we look at the object without binoculars.
When we talk about the objective lens diameter (like 42mm or 50mm) we are also indicating the amount of light that the lens captures. The job of the objective lens is to capture light (and then flip and invert it) as it travels through the binocular. A larger objective lens will therefore result in a brighter and sharper image, though mostly in low-light conditions.
Binocular Numbers are Located on the Binocular Housing
The number pairing that describes binoculars can be found on the housing of the binoculars.
These numbers are often located at the top edge of the focus wheel or on one of the binocular barrels. You can see these numbers at the top of the focus wheel in the image on the right. They will also be included in the model’s name (as in “Nikon Monarch 7 8×42” binoculars).
Which Binocular Magnification is Better?
As for which binocular magnification is better, the answer is that it depends. Let’s have a look:
When 10x42 Binoculars Do "Better" Than 8x42 Binoculars |
Perceived Closeness of an Object: Objects look closer with more magnification. |
Twilight Factors (detail in lowlight): In lower light, at higher magnifications, images display more detail |
Image Detail at Longer Distances: When zooming in from a distance, at higher magnifications we can see more detail |
When 8x42 Binoculars Do "Better" Compared to 10x42 Binoculars |
Image Stability: There is less image shake at the lower magnification. |
Field of View: We can see more of the image at the lower magnification. |
Image Brightness/Low Light Performance: Images look brighter at the lower magnifications. |
Likelihood of Vignetting: The exit pupil is larger at the lower magnification and easier to align with eye pupils. Therefore, there is less risk of vignetting |
Wildlife Scanning at Close- to Mid-Distances: It's easier to scan for wildlife at the ower magnifications at close-to-mid distances since you have a wider field of view. |
Ease of Following Fast Moving Target: It's easier to follow fast moving targets at the lower magnification at close-to-mid-distances. |
Number of Animals Seen in Group: At the lower magnification, you will see a greater number of individuals in a group. |
Weight: Lower magnification binoculars are ever so slightly larger. |
Time To Focus: It takes less effort, and so less time, to focus at the lower magnification. |
Eye Relief: You get more eye relief at the lower magnification. |
Close Focus Distance: At the lower magnification, the distance you need to be from an object while still focusing on it is less. |
Price: Binoculars with lower magnification tend to cost less. |
We can see that as the binocular number for magnification increases (that is, the bigger 10x magnification on the 10×42 binoculars compared to the lower 8x magnification on the 8×42 binoculars) various factors are impacted in positive and negative ways.
When Higher Magnification is Better
With higher magnifications, objects appear to be closer to us, we can see more of their detail during twilight hours, and, at greater distances, we can see more detail when we zoom in.
Objects Look Closer at Higher Magnifications
At higher magnifications, objects appear to be closer to us. Let’s use an 8×42 binocular as an example. The first binocular number – “8x” – means that the object will appear 8 times closer than it would if you were viewing the object without binoculars.
For a great video-based answer to the question “What Do The Numbers On Binoculars Mean?” check out the excellent video below:
For example, if we are focusing on the details of a bird’s bill with an 8×42 binocular, this means that the bill will appear 8 times closer than it would if we were viewing the bill without a binocular.
An object viewed with a 10×42 binocular will appear 10 times closer.
Keep in mind that the diameter of the objective lens has no impact on magnification. So, for example, both an 8×42 and an 8×32 binocular (where the objective lens diameter numbers – 42mm and 32mm – are different, but the magnification number – 8x – is the same) will both magnify an object so it appears 8 times closer than it normally would.
But what does this really mean when we think about birds viewed at specific distances?
Imagine a Robin, sunning itself on a fence post. If the Robin is 80 feet away, then viewing it with a 10x binocular will make it appears as though it is only 8 feet away. Viewing it with an 8x binocular will make it appear as though it is 10 feet away.
During Twilight, Images Look More Detailed at Higher Magnifications
At higher magnification, we can see more detail in lower lighting conditions, such as dawn or dusk. To calculate Twilight Factor, multiply the objective lens diameter, in mm, by the magnification power, and then take the square root of that value. The larger the value, the more detail we can see in low light.
This finding may seem counterintuitive since lower magnification numbers result in a brighter image (as discussed in the “Cons” section below). However, detail and brightness are not the same: even though the image produced by an 8x binocular is brighter, we can still see more image detail with the 10x in low light.
From Long Distances, Images Look More Detailed at Higher Magnifications
Higher magnification numbers allow us to see more detail when we view an object from a longer distance. This is because we can zoom in on details more than we can when using binoculars with a lower magnification.
When Higher Magnification is NOT Preferred
At higher magnifications, there is more image “shake,” less field of view, less image brightness/low-light performance, more chance of vignetting, and difficulty scanning at close- to mid-distances.
There is also the ability to see fewer individual objects in a group, a slight increase in binocular weight, more effort/time to focus, poorer close focus distance, and a higher price point.
Images Shake Less at Lower Magnifications
Image stability is best at lower magnifications, but at higher magnification numbers, stability becomes an issue. As we go higher up the magnification scale, even small hand movements will result in significant shaking of the image, as every movement will be magnified.
This is especially difficult when you are using one hand or if both hands are unsteady. Walking while using high-magnification binoculars will be even more problematic. Focusing on holding the binoculars steady will get old very quickly.
For optimal stability, it’s best to keep magnification at 8x or below. Anything above that should be used with a tripod.
Field of View is Wider at Lower Magnifications
Field of View refers to how much of a view you can see from left to right. It is usually notated in a linear value, such as how many feet (meters) in width will be seen at 1,000 yards (or 1,000 m), or in an angular value of how many degrees can be viewed.
The field of view increases at lower magnifications and you can see more of an image. Conversely, with higher magnification numbers, the field of view decreases so you see less of the image. The widest field of view is achieved with the naked eye. Any binoculars will decrease the field of view, and those with higher magnifications will decrease it the most.
So, for example, if you are viewing a scene without binoculars, like the one depicted below, on the far left, you can see the two turquoise pools, as well as the clouds above it.
Now let’s say we now view the scene with a pair of binoculars that have an 8x magnification. Remember that this means that objects will appear 8 times closer than if you were viewing the object without binoculars. What happens to the field of view?
If we look at the center image, we can now see more of the details of the waterfall and the bottom turquoise pool, but we can only see the right edge of the left pool and can barely see the sky.
If we increase the magnification even further to 12x, for example, the narrowing of the
field of view is even more restricted: As shown in the right-most illustration below, both pools are completely obscured – but we get a great close-up of the waterfall.
Trade-off Between Magnification vs Field Of View
In summary, with higher magnification numbers, there is a trade-off: as we increase magnification, even though what you can see will appear much closer, you will see less, overall, since the field of view won’t be as wide.
Conversely, as we decrease magnification, the field of view expands – we see more but at a lower magnification. We also see images that are a bit darker.
But what does this mean for birding? Is it better to be able to view a bird at a higher magnification, with a more restricted field of view, or is a lower magnification, with a wider field of view more desirable?
At close- to mid-distances, a wider field of view is especially desirable since this will also allow you to more easily spot a bird resting amongst large sections of foliage and bramble.
Since birds are often in motion, a wider field of view will allow you to track them as they flit from branch to branch, swooping from tree to tree.
However, if your birdwatching activities consist of viewing a raft of slow-moving ducks, for example, at longer distances over broad, empty spaces, where foliage is not an issue, and zooming in for more detail, a higher magnification may be more desirable.
Images Look Brighter at Lower Magnifications
At lower magnifications, images are perceived as brighter, but with higher magnification numbers, we get less image brightness. This is due to the size of the exit pupil which refers to the width of the light beam that exits the ocular lens and falls on your retina. The Exit Pupil is lower (and so the light beam is narrower) when magnification is higher.
A duller image is the last thing you want when viewing that magnificently colored male Northern Red Cardinal, the cerulean blue of a Jay, the beautifully patterned Tawny Owl, or the iridescent-green head and yellow bill of a male Mallard.
To calculate the exit pupil, divide the lens diameter, in mm, by the magnification.
Deep Dive on Exit Pupil
To utilize the full amount of light from the Exit Pupil, it should be at least as large as the size of your eye’s pupil.
In daylight, the difference in exit pupil size between the two binoculars won’t make much of a difference because your eye’s pupil size will be smaller than the exit pupil in both the 8x and the 10x, for example.
In contrast, in low light conditions, like dusk or dawn, your exit pupil will dilate so they can capture more light. In this case, the 8x’s larger exit pupil will definitely produce a brighter image since it will be larger than the size of your dilated eye.
Interesting fact: In low light, the pupils of kids’ eyes dilate more than those of adults. This means that children’s eyes can capture more light and so their image will be bright.
In addition, even if the size of the exit pupil is larger than the eye pupils of both kids and adults, the image the child sees will be brighter because they have bigger pupil dilation than adults do. So, there is more pupil to capture and utilize more light.
Vignetting Decreases at Lower Magnifications
Vignetting looks like a dark shadow around the edges of the image. It is due to the fact that the edges of the field of view are not as bright because the eye is not lined up with the exit pupil, or because the exit pupil is smaller than the eye pupil.
At lower magnification, the risk of vignetting decreases. There is more chance of vignetting with a higher magnification number due to the size of the Exit Pupil which, again, decreases when magnification increases. This means that the light beam exiting the ocular lens will be narrower.
A smaller exit pupil (narrower light beam) will require more precision when aligning the pupils of your eyes with the exit pupil. If the exit pupil is not larger than your eye pupil, there will not be any overlap or wiggle room.
And if your eyes and exit pupil are not aligned, you will not see the full image, and vignetting will occur. When an exit pupil is larger than the size of your eye pupil, it is easier to position your eye in the location of the exit pupil without being precise. You will be able to easily obtain the full field of view.
Scanning at Close- to Mid-Distance is Easier at Lower Magnifications
At lower magnifications, it is easier to scan for objects at close- to mid-distances. Conversely, it is harder to scan for active creatures with a higher magnification number when we are viewing them at a relatively close to mid-range distance. This is because, with higher magnification, the field of view is narrower.
With a narrower field of view, it is harder to scan for, spot, and follow an active songbird, flitting from branch to branch amongst dense foliage, or a barred owl who takes off the moment he detects your presence.
We Can See More Objects in a Group at Lower Magnifications
At lower magnifications, when we observe a group of birds, for example, we can see more of them since the field of view is wider. We can see fewer objects in a group as the magnification number increases since the field of view is narrower. For example, we can see fewer individual birds in a flock, and fewer individual deer in a herd.
Binocular Weight is Slightly Less at Lower Magnifications
At lower magnification, binoculars are slightly lighter. As magnification increases, binocular weight increases because the objective lens is a bit thicker at higher magnifications.
Even though relatively petite people may notice the difference in weight over the course of a few hours, it may not be noticeable to people who are larger.
Focusing Takes Less Effort and Time at Lower Magnifications
At lower magnifications, it is easier and less time-consuming to focus on an object. But at higher magnifications, it will take more time and effort to focus on them.
This is because focusing on closer objects will require more turning of the focus wheel. The extra time involved may mean losing sight of that mighty barn owl.
By the time you’ve focused, he has flown across the pond and disappeared from view.
Eye Relief is Longer at Lower Magnifications
At lower magnifications, the eye relief length of a binocular is longer. With higher magnification numbers binoculars have shorter eye relief. This factor is of great relevance for binocular users who wear glasses since longer eye relief is a must.
We Can Focus on Closer Objects at Lower Magnifications
Close focus distance refers to how close you can be to an object and still focus on it. For example, a close focus distance of 6 feet means that you can clearly focus on a butterfly that is as close as 6 feet to you.
Higher magnifications compromise close focus distance. For example, when viewing a butterfly with an 8x binocular you will be able to view its details from a closer distance than if you were using 10x binoculars.
The close focus distance is often of interest to birders since, in addition to focusing in on birds from afar, they may have opportunities to view birds up close as they splash in a garden fountain, sun themselves on a deck ledge, or nibble from a bird feeder.
Binoculars with low close-focus distances allow for zooming in on intricate features, such as bird beaks and markings. For birders who are also interested in viewing other forms of nature a lower close focus distance is advantageous as well. Many birders I know also enjoy observing smaller critters such as salamanders, bees, and frogs.
The sweet spot for both magnification and close-focus viewing is binoculars with 8x magnification.
Price is Generally Less Expensive at Lower Magnifications
At lower magnifications, binocular prices are a bit more affordable. Unfortunately, binoculars with higher magnifications are generally more expensive than binoculars with lower magnifications.
What Magnification Does Not Impact
8x and 10x Binoculars Weigh About the Same
Magnification does not significantly impact binocular weight. For example, an 8×42 and 10×42 binocular are pretty much the same weight, with the 8×42 being a tad heavier due to the slightly thicker objective lens of the 10x.
A few petite users may perceive this admittedly almost imperceptible difference over a few hours of use, but those with larger builds probably will not. This slight difference is indicated by the lighter green checkmark in the 8×42 column in the above table.
8x and 10x Binoculars are Close in Size
Magnification also does not impact binocular size. Again, if we look at an 8×42 and a 10×42 pair of binoculars, they are virtually indistinguishable.
To check out the actual measurements and weights of binoculars with different magnification numbers having the same size objective lens, let’s look at the Viper HD specification table below.
If we look at the third and fourth rows, we can see that the Vortex Viper 10×42 and 8×42 binoculars both have the same overall height x width of 5.6” x 4.9” (see last column,) and a small difference in weight (24.5 oz vs 24.9 oz.)
This is also the case for the measurements and weights of the 12×50 and 10×50 displayed in rows 1 and 2 – no difference in height and width but a slight difference in weight.
What is the Impact of Binocular Lens Diameter?
In general, as the size of the objective lens diameter number increases, binocular size, and weight increase, the quality of the image is bigger and sharper, and the size of the light beam that exits the objective lens and makes its way through the binocular is larger.
A Larger Objective Lens Diameter Increases Overall Binocular Size
Unlike binocular magnification, the number of the objective lens diameter number significantly impacts the overall size of the binoculars – a larger objective lens will increase the overall binocular dimensions.
A binocular described as an 8×42 has an objective lens diameter of 42mm (1.65 inches,) while 10×50 objective lenses are 50 mm (1.97 inches) in diameter.
Overall binocular sizes, and their respective objective lens diameters, are often divided into 4 categories.
Common Binocular Categories:
- Compacts: 21mm-28mm
- Mid-Size: 30mm-34mm
- Full-Size: 42mm-50mm
- Large-Size: 60mm or more
A Larger Objective Lens Diameter Increases Binocular Weight
As already noted, the number of the objective lens diameter significantly impacts the overall size of the binoculars – a larger objective lens will increase the overall binocular dimensions, making them heavier as well.
A larger, heavier pair of binoculars is not ideal for holding binoculars or toting them in a pack. Heavier binoculars might be OK for short birding jaunts, but after several hours can become very uncomfortable.
A Larger Objective Lens Diameter Increases Image Quality
In addition, just like viewing an object through a large window, viewing an image through a larger lens diameter also increases the perceived brightness and sharpness of the image. This is because larger lens diameters have a larger aperture, and so can capture more light.
However, the number of the objective lens diameter and the impact it has on image brightness is most noticeable in low light conditions, such as dawn and dusk, and less so during bright daylight hours. In addition, the brightness of the image will also be determined by the quality of the lens and prism glass and coatings, since these can reduce light loss.
A Larger Objective Lens Diameter Increases Image Brightness
Why does a larger objective lens diameter increase image brightness? To understand this, we first need to understand the role of the exit pupil: A larger front lens produces a wider beam of light that travels from the objective lens and transits the interior of the binocular. When the light beam exits the eyepiece (lenses closest to the user’s eye,) and makes its way to the pupil of the user’s eye, the width of the beam (called the exit pupil) impacts perceived image brightness.
Ideally, we want the size of the exit pupil to be at least as large as the size of the eye pupil, if not more so.
What Do The Numbers On Binoculars Mean? – It’s a Balancing Act
Understanding binocular numbers can be a bit tricky but hopefully, you now have a much firmer grasp on the meaning of binocular numbers.
So, how do we balance all of these seemingly conflicting needs? How much magnification is enough, and can we get there without compromising the field of view, eye relief, brightness, weight, size, and stability?
If you are an eyeglass wearer who is searching for an excellent and affordable pair of binoculars, be sure to use this checklist.
It’s a balancing act and also depends on the nature of your birdwatching activities.
Ideally, when searching for the best binoculars for birding, we are looking for binoculars with the following characteristics:
- Objective lens diameter size that is large enough to provide us with a bright, sharp image
- A binocular size that is easy to hold and not too heavy
- A magnification that significantly increases the image of all creatures great and small
- A wide field of view for scanning the horizon
- Less time to focus so we can quickly zero in on our fine-feathered friends
- Excellent eye relief (critical for those who wear glasses)
- Image stability for less movement when we move
- Impressive focus distance for focusing in on butterflies and bees
Taking all of these factors into account, it turns out that binoculars with a magnification of 8x and 10x offer the best compromise solutions for birdwatching, with the 8x binocular being the most popular choice.
In fact, if you are investing in one great all-around pair of daylight birding binoculars, plan to view fast-moving birds from close- to mid-distances in parks and woodland, where foliage is thicker, the 8×42 binoculars are the way to go.
However, if you plan on using a tripod, want maximum magnification while still achieving stability, and plan to do much of your birdwatching at longer distances, across wide empty expanses, perhaps viewing slow-moving water birds, then 10×42 binoculars are a great choice as well.
So, if you want to know which binocular magnification is better, it really depends on who you plan to use your binoculars.
So now, if someone asks you “What do the numbers mean on binoculars,” you should hopefully be able to answer the question like a pro!
FAQ
What is the field of view on binoculars?
The field of view on binoculars is the width of the viewable area and refers to how much of a view you can see from left to right. It is usually notated in a linear value, such as how many feet (meters) in width will be seen at 1,000 yards (or 1,000 m), or in an angular value of how many degrees can be viewed.
Is 8×42 better than 10×42?
8×42 binoculars are the preferred choice of most birders. However, birders who use tripods, or birdwatch from a greater distance, and in a large open area may prefer 10x42s.
Is a higher field of view better for binoculars?
Since birding may consist of viewing a flock, spread out over a larger area, a wider field of view is advantageous. A wider field of view will also allow you to more easily spot a bird resting amongst large sections of foliage and bramble.
Finally, since birds are often in motion, a wider field of view will allow you to track them as they flit from branch to branch, swooping from tree to tree.
Why does the objective lens have a large diameter?
The objective lens has a large diameter because it collects light from objects. Just like viewing an object through a large window, viewing an image through a larger lens diameter increases the perceived brightness and sharpness of the image. This is because larger lens diameters have a larger aperture, and so can capture more light.
Is a bigger objective lens better?
A bigger objective lens is better on binoculars because the bigger the objective lens, the more light it collects and the brighter and sharper the image will be. However, as the objective lens size increases, so too does the overall size of the binocular and its weight.
Many birders prefer a 42″ objective lens diameter since it has great light-collecting power while still being small enough so that the overall size of the binoculars and their weight is not significantly impacted.
