What Is Exit Pupil In Binoculars?

The short answer is, the exit pupil is the circle of light that you can see when you hold binoculars at arms length and look through the objective lens. Of course there’s a whole lot more to it than that, but that gives you something to think about as we fill in the details. The exit pupil is an important yet, often overlooked feature on a pair of binoculars.

We all look for the obvious features like the magnification level and the size of the objective lens diameter and even the field of view, but who thinks about the exit pupil? Yet it’s the only way light has of entering the binocular lenses. And therefore, the only way to illuminate the image we can see through the binoculars. Under certain conditions the size of the exit pupil could be the difference between seeing anything at all through the binoculars or not.

What Has The Exit Pupil To Do With The Human Pupil?

The human pupil, our pupil, reacts with the light, in bright sunlight our pupils shrink to prevent too much light from entering our eyes. As darkness falls, our pupils dilate to allow more light to enter our eyes so we can still see in darker conditions. There’s an average rate at which the human eyes’ pupil size decreases with age.

An average human pupil at rest in dull conditions will be around 7mm at the age of 30 years old. The pupil size decreases by around one mm every decade so at 40 it’s 6mm and so on until we reach the age of 80 when it settles at around 2 to 2.5mm. In bright conditions our pupils are around 2.5 to 3mm, but as darkness falls our pupils increase in size, to the level quoted above depending on our age.

Average Pupil Size By Age

Age	Daytime Pupil Size	Night Time Pupil Size
20 years old	4.7mm	8.0mm
30 years old	4.3mm	7.0mm
40 years old	3.9mm	6.0mm
50 years old	3.5mm	5.0mm
60 years old	3.1mm	4.1mm
70 years old	2.7mm	3.2mm
80 years old	2.3mm	2.5mm

To see clearly through binoculars we need enough light to hit our pupil to illuminate the image we’re looking at.This means we need an exit pupil of as close to our own pupil size to use binoculars correctly, especially in low light conditions. The exit pupil isn’t always quoted in the specs when buying binoculars, but it’s easy to calculate with just one simple formula.

How To Calculate The Exit Pupil On Binoculars

As long as you know the magnification and the objective lens diameter you can calculate the exit pupil of any pair of binoculars. Just divide the objective lens diameter by the magnification, the result is the exit pupil in mm.

The Exit Pupil = Objective Lens Diameter ÷ Magnification

Take as an example a pair of binoculars with the following numbers 7×50 That’s a magnification of 7 times (7x) with an objective lens diameter of 50mm. Using our exit pupil formula, we get;

50 ÷ 7 = 7.14

That means the exit pupil of any binoculars with the same numbers (7×50) will have an exit pupil of 7.14mm. 8×42 binoculars will have an exit pupil of 5.3mm, 10×50 will have an exit pupil of 5mm and so on.

The greater the magnification, the smaller the exit pupil and the larger the objective lens diameter the larger the exit pupil. Both the magnification and the objective lens diameter have an effect on the size of the exit pupil. 

What’s The Connection Between Exit Pupil And Eye Relief?

There is a connection between exit pupil and eye relief but before we get into that, let’s explain what eye relief means. 

Eye Relief

Eye relief is the perfect distance between the outer surface of the ocular lens (the lens closest to your eye) and your eye and still see the full field of view. The eye relief is where all of the light from the exit pupil converges into focus and forms the complete image. The average eye relief is around 12 to 16mm but if you wear glasses you’ll probably need what’s known as long eye relief.

Long Eye Relief

Long eye relief is a greater distance between the ocular lens and your eye, allowing for the extra space needed for your eyeglasses while still keeping that perfect distance between the ocular lens and your eye. Long eye relief is usually anywhere between 16 to 24mm.

The Perfect Spot For Full FoV

Your eye should be positioned just behind the eyepiece to experience eye relief. If your eye is too far away from the eyepiece, you’ll lose the FoV, and you could move your eye away from the light beam from the eyepiece. Having your eye too close to the eyepiece will not allow you to blink and you’ll also notice a black ring framing the FoV, thus depriving you of the full image.

Age And Exit Pupil

As we age, our pupils dilate less in dark conditions (due to our pupils shrinking by one mm per decade) this means as we age, compact binoculars like 8×32 seem to be the perfect fit for older people. By the time we reach 50 years of age, our pupil size will be around the 5mm mark, which means binoculars with a 5mm exit pupil, that have a smaller objective lens diameter and a smaller exit pupil will present us with an image just as bright as larger, heavier and often more expensive binoculars.

What Does Exit Pupil Mean To Stargazers That Use Binoculars?

This is where things can get slightly confusing, so bear with us, we’ve so far discovered that the exit pupil is determined by the relationship between the magnification and the objective lens diameter. So when we say a certain pair of binoculars has a small exit pupil, we’re really saying they have a higher magnification.

But does higher magnification create a dimmer image? Take for example the Andromeda galaxy. When looking at galaxies or even nebulae, there are no binoculars that will make the image brighter. Easier to see yes, but brighter, no. What’s the reason for that? The answer is, magnification.

The magnification makes the galaxy appear through the binoculars to be larger, and to a certain degree, the more magnification used, the bigger the galaxy will appear and the easier it will be to see it. Which is why 10×50 binoculars give us a clearer view of the galaxy than 7×50 binoculars even though they have a smaller exit pupil.

Most stargazers avoid binoculars with big exit pupils because they show the background at its highest brightness. However, using higher magnification causes both the galaxy and the background sky to be dimmed to the same amount. In other words, the contrast between the galaxy and the surrounding sky remains the same. The darker background sky and the larger scale of the image, appears to improve the contrast, even if galaxy brightness in relation to the surrounding sky hasn’t changed.

What About Stars?

When Talking about looking at stars through binocular lenses, it’s a different story. When we look at stars using higher magnification (thus smaller exit pupils) produces a dimmer background view but the brightness of the star is fixed by the diameter of the objective lens. Put clearly, when looking at stars, increased magnification dims the surrounding sky but not the star itself. 

How Does This Work?

It’s because there isn’t a high enough magnification to make them appear any bigger. All of the stars’ light remains concentrated into a pin point. By magnifying the galaxy, it appears to increase in size and the light it produces spreads over a wider area. Due to this boost in contrast, as the magnification increases, fainter, duller stars become easier to see.

For Stargazing A Smaller Exit Pupil Is Better

Smaller exit pupils give better views of galaxies due to the increased darkness of the background sky. That darker background gives stars an improved contrast, with larger looking fainter stars appearing larger and easier to see. This is exactly how telescopes operate too.

So you need to choose between using lower or higher magnification. Lower magnification means a wider field of view, so if seeing more with less detail is what you want, choose lower magnification. If it’s more important for you to see more detail and lose some of the FoV, then higher magnification will suit you better.

How Does Exit Pupil Affect Vision With Binoculars?

To sum all of this up, the human pupil is around 7mm in the dark and around 2mm in bright daylight. Binoculars with an exit pupil of more than 2mm during daylight, will show no dark images, but using binoculars with a larger exit pupil during the day won’t make the images appear any brighter.

However, it’s at night when it all changes, using binoculars with a small exit pupil in the darkening conditions, will result in a dull image. In fact under such conditions you’ll probably see more with the naked eye. So as natural light fades, you’ll need to use binoculars with a larger exit pupil to make out any detail of the object you’re trying to view.

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