If you are an amateur astronomer or just interested in stargazing, then you have probably heard of the exit pupil. But what is an exit pupil and how does it affect your binoculars? This guide will explain everything you need to know about exit pupils in binoculars, from what it is to how it affects your viewing experience. After reading this guide, you will have a better understanding of how to choose the right binoculars for your needs.
So, what is an exit pupil in binoculars?
An exit pupil is the beam of light that comes out of the eyepiece of your binoculars. It appears as a small, round spot of light against a dark background. The size of the exit pupil determines how much light is available to your eye and how bright the image will appear.
A larger exit pupil means more light and a brighter image. On the other hand, a smaller exit pupil means less glare and a dimmer image. As the human eye can only use so much light, there is no need for an exit pupil more significant than 7mm. Most binoculars have an exit pupil between 2mm and 5mm. However, for a comfortable viewing experience in any outdoor activities like birding, hiking, sporting events, wildlife viewing, etc, it should be more than 4.00mm
Moreover, the exit pupil also affects how easy it is to hold the binoculars steady. A larger exit pupil requires less effort to keep steady, while a smaller exit pupil requires more effort.
How to measure binoculars’ exit pupil?
Binoculars are considered one of the most commonly used tools for stargazing. But how do you know which binoculars are right for you? Well, one important factor to consider is the exit pupil. Let’s see how to calculate it.
You can see two numbers on the binoculars 10×42, 12×50, 8×32, and so on. The 1st value is the magnification power of the particular bino pair. As a result, you can see magnified images on this scale with these binoculars. The 2nd value defines the size of the objective lens. This size determines how much light can be collected by these binoculars. And you can easily measure the exit pupil value by this magnification and objective lens ratio.
To calculate the exit pupil distance of a binocular, you need to divide the size of the objective lens by the magnification power. For instance, if you have a pair of 10×50 binoculars, the calculation would be 50 / 10 = 5.0mm. If you choose an 8×42 binocular, the exit pupil will be 5.25mm.
Why is the exit pupil important?
We all know how important it is to ensure our vision is as clear as possible. After all, our eyesight is one of the most important senses we have. But did you know that the size of exit pupils of binoculars can affect your vision?
Viewing through binoculars can be tiring on the eyes if the exit pupil is not large enough. The exit pupil is the diameter of the amount of light exiting the eyepiece. This needs to be large enough, so your eye can relax while viewing. A small exit pupil will cause your eye to work harder, and you will likely experience eye fatigue.
In addition, the exit pupil is important in binoculars because it is the image of the objective lens that is seen through the eyepiece. The size of the exit pupil determines how much light is gathered by the binoculars and how much detail and brightness can be seen.
Exit pupil in daylight and low light condition
Research says, from age 20 to 50 years, the eye pupil diameter is between 4.7 to 3.9mm in daylight. For the same people, it will increase at night time. And the range of eye pupil diameter will be 8mm to 5mm according to age.
Similarly, binoculars are designed to work in a variety of lighting conditions, it’s important to understand how bright/low-light conditions can affect a binocular’s exit pupil.
In bright conditions, a binocular exit pupil will be smaller than in low-light conditions. This is because the human eye can only dilate so much in bright light. As a result, a binocular with a large exit pupil may appear to have a “tunnel vision” effect in bright light.
On the other hand, in low-light conditions, a binocular exit pupil will be larger. This is because the human eye can dilate more in low light, allowing more light to enter the eye. As a result, a binocular with a large exit pupil will provide a brighter image in low light.
It may seem a little bit tricky. So here I am going to depict this scenario by using examples. Hopefully, you can understand it much better.
In bright condition
For instance, you have 2 bino pairs of 7×50 and 10×32. If we consider the 7×50 binocular, it will provide you with (50/7) = 7.1mm exit pupil. On average the eye pupil diameter of human eyes one day is 2 to 4mm. So here we can see that the exit pupil of binoculars is greater than the eye pupil of our eyes. As a result, our iris will block half of the lights and they won’t be able to reach our retina. Though it’s in daylight condition, so you will get the same brightness in the magnified images as your nacked eyes. Also, it will ensure a smooth and comfortable viewing session too.
On the other hand, if you use the 10×32 binoculars that will deliver you (32/10) = 3.2mm exit pupil distance. This value is almost similar to our average eye pupil diameter in daylight conditions. At this time, you will also get a bright image like your naked eyes. If the value is less than 4mm, that would not be recommended even if you can see the bright output of the long-distance objects considering the viewing comfort.
In low light condition
In the nighttime, human eye pupil diameter is increased and 7mm is regarded as the average value. If you use a 7×50 binocular, it will provide a 7.1mm exit pupil value. It’s quite the same with the eye pupil size, so the binocular can collect enough light and can produce bright images.
Again, if you choose the 10×32 binoculars for nighttime or low light conditions, the distant objects will appear darker. Also, it will cause eye strain as your eyes don’t get enough light.
Note – When choosing a binocular, it’s important to consider the type of conditions you’ll be using it in. If you’ll be using it mainly in low-light conditions, such as at dawn or dusk, then a binocular with a large exit pupil is a good choice. If you’ll be using it mainly in bright conditions, such as during the daytime, then a binocular with a smaller exit pupil is a better choice.
Frequently Asked Questions(FAQs)
Is exit pupil the same as eye relief?
No, the exit pupil is not the same as eye relief. The exit pupil is the diameter of the light beam that enters your eye. Eye relief is the distance between your eye and the eyepiece lens.
What is a good exit pupil for binoculars?
Exit pupil distance should be more than 4.00mm to get a relaxed and smooth viewing experience without experiencing eye fatigue. For low light conditions, it should be around 7mm.
Is Long exit pupil better?
Yes, a large exit pupil will give your eye more time to adjust to the light coming in. This is especially important when viewing objects that are far away or have low contrast.
In conclusion, knowing about the exit pupil in binoculars is much more crucial. Because it determines how much light enters your eye, and therefore how bright the image will be. Also, for a smooth and relaxed viewing experience, choosing the binoculars with the right exit pupil is a must. Hopefully, now you can easily measure the value by the magnification and objective lens ratio.
Hopefully, this entire discussion will help you choose binoculars that will give you the best possible view.