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What Is Exit Pupil on Binoculars?

The exit pupil is the size of a light-catching area on an optical instrument. But what is exit pupil on binoculars? And why is it necessary?

When you’re choosing binoculars, one of the most important factors is the optic pupil. It’s actually the size of the lens in your binoculars that lets bright light escape from the device and is indicated in millimeters.

The exit pupil of binoculars must match the viewer’s entrance pupil. It prevents vignetting by properly coupling the optic system with the eye. Also, it affects how well you see details in fainter objects.

Whether using binoculars for bird watching or outdoor sightseeing activities, you should know the size of an object’s exit pupil will help you see better without straining to focus on distant objects at deep twilight.

This detailed guide will help you understand more about this and other relevant factors to enrich your far-sight ability.

Why Is Exit Pupil on Binoculars or Telescopes Important?

It’s a must to know about the key role of exit pupils for beginners, professional hunters, and explorers before and after buying an expensive binocular. It determines the distant data of the object you’re looking at.

As a result, you can identify whether the target is moving or what should be your next step to get a bull’s eye.

The larger the size of your exit pupil, the more light will be transmitted to your eyes. This means you’ll see clearer images, and images will appear sharper.

A large exit pupil is especially important if you’re using binocular eyepieces for hunting or observing wildlife because it lets in lots of light than smaller apertures. It also means you witness less lens glare and a better view in low-light situations.

Binoculars with large exit pupils are better for viewing close targets. On the contrary, smaller exit pupils are best for viewing distant objects.

Having these understandings will help with various purposes, whether you’re camping for a target or just exploring the wild mountain.

What Is the Ideal Size of the Exit Pupil of Binoculars?

If you are a huntsman or bird watcher, the size of your exit pupil is crucial. The exit pupil is the smallest part of binoculars that allows light to pass through. It’s measured in millimeters and has a disk shape at the front end of each eyepiece.

The larger the exit pupil, the better resolution you’ll witness when seeing an object because more light will pass through to your eye pupils.

However, the ideal size of the exit pupil is a topic of much debate and research. There are many factors to consider when choosing an exit pupil, such as the human eye pupil lenses, the effective diameter of the distance, the type of light fixture, and the size of the viewer.

Some experts say that an exit pupil should be as small as possible to reduce eye fatigue, while others argue that a larger exit pupil creates a more immersive experience. It’s crucial to find what works best for your particular project or environment.

·        Average Ideal Exit Pupil Diameter on Binocular

The minimum expected exit pupil size for most hunting optics is 6 mm, and for birding visuals, it ranges from 4 to 6 mm. For binoculars with magnification powers of 10x, a maximum pupil diameter of 8 mm is optimal for most users.

However, some users may benefit from 10 mm, or even 12 mm exits pupils, depending on their individual preferences and needs.

·        Recommended Exit Pupil Size

Human eyes are actually quite interesting. In the daytime, the human pupil widens around 2mm. While seeing objects at night or in a dimmer light, it opens around 7mm when seeing objects.

Having a similar diameter of exit pupil on your binocular is also preferable. The similar dimension passes enough lights through your eye so that you can get enough details of the views.

How to Calculate the Size of the Exit Pupil for Binoculars?

Pupil size changes according to the amount of light entering our eyes. As mentioned earlier, the human eye’s pupil change size according to the brightness of the environment. Pupil size decreases when there is less light.

The exit pupil diameter is the light beam size that leaves the eyepiece of a binocular or telescope. It can be determined by dividing the lens diameter by the telescope’s magnification.

This calculation is important for determining how much light reaches your eye. A larger exit pupil diameter allows more light to reach your eye, which results in a brighter image.

You can calculate the size of the exit pupil for binoculars by dividing the aperture of the objective lenses by the magnification of the binoculars. It’ll give you the diameter of the exit pupil in millimeters.

 

Exit Pupil Calculation Formula –

  • The aperture of binocular lens ÷ magnification power of binocular = Exit pupil of the binocular

For example, assume you’ve got a magnification of 10X and a size of 1.0m for the mountain peak.

Therefore, the distance from our eyes to the mountain peak is:10x ÷ 1 = 10m

This means that the mountain peak is about 10 meters away from us. When using a telescope instead of binoculars, it becomes much easier to estimate the distance to the mountain peak because telescopes have very high magnifications.

 

Why There’re Different Sizes of Exit Pupils on Binoculars?

The reason you see different sizes of an exit pupil on different compact binoculars is due to the following factors and purpose:

  • magnification of the binoculars
  • the brightness of the target
  • distance between the observer and the target
  • the angle at which the binocular is held relative to the target (i.e., whether it’s held horizontally or vertically)

 

Minimal vs. Maximal Exit Pupil – What’s the Difference?

There are two main types of exit pupils: minimal and maximal. A minimal exit pupil is the smallest possible point of light that can be seen through the eyepiece of a telescope or camera.

A maximal exit pupil is the largest possible point of light that can be seen through the eyepiece.

The size of an exit pupil has a significant impact on how well you’ll see celestial objects. If your telescope’s exit pupil is too small, you will not be able to see any detail in the objects you are looking at.

If your telescope’s exit pupil is too large, you’ll be able to see only a part of an objective diameter, and the rest of the part will seem blurry.

 

How to Calculate Exit Pupil on Different Binoculars?

Depending on the binocular advantage of magnification size and human pupil size, the distance may differ. The size of an exit pupil also depends on the type of binoculars used.

For more information on calculating distances with binoculars, read the below chart –

 

Binocular Type
Pupils Diameter of Human Eye
Formula (Distant/Pupils)
Exit Pupils of Binoculars
9×42 4.5mm 42 ÷ 4.5 9.3mm
10×50 5mm 50 ÷ 5 10mm
11×50 5.6mm 50 ÷ 5.6 8.9mm
12×60 6.2mm 60 ÷ 6.2 9.7mm
13×70 6.5mm 70 ÷ 6.5 10.8mm
14×80 6.8mm 80 ÷ 6.8 11.8mm
15×100 7.2mm 100 ÷ 7.2 13.9mm
16×120 7.4mm 120 ÷ 7.4 16.2mm
17×150 7.7mm 150 ÷ 7.7 19.5mm

 

Note: High-quality binoculars with large exit pupils have a brighter image than binoculars with small exit pupils. However, this does not mean that they produce better images. It only means that their images appear brighter because they have a larger area for the light rays coming through the exit pupil to fall onto your retina.

 

What Affects the Exit Pupil Size on Binoculars?

There’re some other factors also in binoculars that influence exit pupil size as well as the other optical mechanisms such as the focal ratio, entrance pupil, lens lengths, inefficient roof prisms, etc. For example –

  • 9×42 binoculars have a larger exit pupil than 6×30 binoculars.
  • 10×50 binocular has a large exit pupil compared to 7×50 binoculars.
  • 11×50 binocular has an even larger exit pupil than a 10×50 binocular. The difference in the exit pupils of these two binoculars is due to the fact that the 11×50 binoculars use a wider objective lens. So, they result in a longer focal length.
  • 12×60 binocular has a larger entrance pupil than 10×40 binoculars. The 12×60 binoculars operate on a shorter eyepiece tube, which results in a narrower field of view.
  • 13×70 binocular has a larger entry pupil than 12×60 binoculars. Usually, the 13×70 binoculars use a longer eyepiece tube.
  • 14×80 binocular has a larger pupil than 13×70 binoculars. The difference probably comes from the fact that the 14×80 binoculars use a longer objective lens.
  • 15×100 binocular has a larger aperture than 14×80 binoculars. And the 15×100 binoculars use a longer optical system.
  • 16×120 binocular has a larger eye relief than 15×100 binoculars. This size of binoculars uses a longer ocular prism.
  • 17×150 binocular has a larger size of eye relief than 16×120 binoculars. The difference between both binoculars is the 17×150 binoculars use a longer focal length.

Is Eye Relief Same As Exit Pupil on Binocular?

Eye relief and exit relief are two different measures of binoculars for sightseeing. While the exit pupil is the diameter of the light captured area, eye relief is the room size where you place your eyes to see the subject through binoculars. In short, it’s the distance from the eyepiece lens to your eye placement.

It’s another key factor you should consider when buying binoculars. For binoculars that are meant for viewing objects near or far, you will want to consider how much eye relief the binocular has.

If you’re looking at something up close and would like to see everything in the wider field of view at once, then a binocular with a long eye relief will give you more room to move your eyes around without having them touch any part of the eyepiece.

On the other hand, if you’re looking at something far away and don’t plan on getting close enough to see any details, then a shorter eye relief binocular may be better for you.

Why Is Magnification Important for Exit Pupils on Binoculars?

It’s important to choose an appropriate binocular or astronomical telescope magnifying power for your viewing location and observing destination so that you can still see all of the detail in dim objects even if their exits pupils are small.

Even though your binocular lens’ diameter is fixed, the variable of magnification indicates to you that its exit pupil can be anywhere between its minimum and maximum diameter.

When using a 2-10×50 scope that has the option to modify the magnification, it can be set between 2 and 10. It is possible to have an exit pupil with a minimum of 5mm, and a maximum pupil size of 25mm.

So, even though you don’t have an adequate exit pupil on your binoculars, you can see the details of the distant object to an extent.

How Does Exit Pupil Work on Binocular Telescopes?

As you know, a binocular has two lenses, one for each eye. When you hold the binoculars up close to your face, the image formed by the left lens falls on the right side of your retina and vice versa. Your brain combines these two images into one clear picture.

Binoculars must send all of the light rays passing through both lenses to your retina to create this single image.

If the exit pupil is too small, some light transmissions will miss your retina altogether. And if you receive less light, it won’t be able to form a clear image, resulting blurred images. In this case, you won’t be able to see what’s in front of you.

This is why the exit pupil of a binocular must be large enough for you to be able to see what’s in front of you clearly.

The diameter of the exit pupil is measured in millimeters (mm). A larger diameter is better. Binoculars with a 10 mm exit pupil can create a clear image of objects as far as 20m away. Binoculars with an exit pupil of 18 mm can create a clear image of objects as far as 50m away.

Editor’s Verdict

In conclusion, an exit pupil is an important specification to look for when purchasing binoculars. The size of the exit pupil on your binocular should be relative to your eye pupil size.

It determines how well you’ll see the details of objects in both low-light and daylight conditions.

Hopefully, this guide regarding what is exit pupil on binoculars helped you understand everything you searched for. Let me know if you also need assistance in buying the best binoculars for distant data.

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By Nathan Moy

Hi, Nathan Moy is the founder and CEO of Birdmoy.com . Im passionate about nature and I use this site as a platform to share my experiences, learnings, mistakes, and ideas about birding and nature.

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