Exit Pupil: What It Is & Why It’s Important for Your Scope

One of my biggest issues when I was a beginner was dark images and poor light conditions.

There were times when I would go outside on a sunny day, but my scope only showed me dim images.

So what was the issue?

Regardless of how long you’ve been using your trusty rifle scope, one of the most important factors to consider is the exit pupil size and quality.

Keep reading to learn more about the functions of an exit pupil!

What Is Exit Pupil on a Scope?

To help explain it a little better, here’s a little exercise I learned.

Try adjusting the scope’s magnification to full, then direct it towards the sky.  Position your eye around 10 inches from the eyepiece and along the optical axis.

You’ll see a bright disc or a bright circle of light in the center — that’s when I learned that that disc is the exit pupil.

When you look through the ocular lens, the exit pupil shows the diameter of light that leaves the eyepiece.

The size of the exit pupil is ALWAYS smaller than the eyepiece and is measured in millimeters (mm).

Entrance Pupil vs. Exit Pupil

Try imagining an optical system that contains various optical apertures that limit the light rays that can pass through.

• The effect of an optical aperture can be changed because of other optical elements
• The position of an optical aperture can affect the opening angle

How do you take all of these aspects into consideration? Here’s where the ENTRANCE pupil and EXIT pupil come in.

Imagine an imaging system with various apertures positioned differently in the beam path. The system is focused on an object plane, and rays start from the center of the object plane.

To solve this, I identify the entrance pupil as the optical image of the aperture. This is seen through the front of the lens system.

The exit pupil (also referred to as a virtual aperture) is the optical image of the virtual aperture. This is seen through the back of the lens system.

How Is Exit Pupil Measured and Calculated?

If I told you that the exit pupil should be calculated on your own, you’d probably be quite hesitant.

But the good news is that you only need to know the magnification and diameter of your lens!

After identifying these, use this simple formula:

(Objective lens diameter in millimeters) / (magnification)

This will get you the estimated size of the scope’s exit pupil. Trust me, it’s simple mathematics!

Let’s say that we’re looking at a 4 x 32 mm rifle scope, and you want to measure the exit pupil’s diameter.

The objective lens is 32 mm, and the magnification is 4x. To calculate the scope’s exit pupil, we’d use the previous formula to get

32/4 = 8 mm

This means that the size of the exit pupil calculated would be 8 mm. I use this formula anytime you want to calculate the size of the exit pupil.

Why Is Exit Pupil Important in Rifle Scopes?

There are various reasons why the exit pupil is one of the most important parts of a rifle scope. These include the following:

• Better image magnification
• Lessens eye strain/protects our naked eyes
• Can make a difference in bright and dark areas
• Takes emergent light into account

Imagine walking out of a dark area into a bright area. You’d automatically raise your hand to protect your eyes, right?

This is because the human eye isn’t accustomed to so much light or so many light rays.

When you find yourself in bright light, your eye’s pupil SHRINKS so that you can see a clearer image. 

When you find yourself in low light conditions, your pupil expands. In the same sense, the exit pupil on rifle scopes serves the same function.

How Does the Exit Pupil Impact Shooting?

When you’re in the center of the woods looking for your next target, you need to have the BEST resources at hand to shoot well.

An important consideration is the objective lens diameter of the exit pupil.

As seen in the formula for the pupil’s diameter, the magnification power of the lens is connected to its size.

A LARGE pupil diameter helps produce a good image in low light conditions.

Meanwhile, a SMALL pupil diameter helps produce a good image in bright conditions.

If your naked eyes are about the same size as the exit pupil, you’d be able to see clear images.

What Is a Good Exit Pupil for a Rifle Scope?

Objectively speaking, a good exit pupil for a scope has the ideal exit pupil diameter.

You may be wondering why the diameter of the exit pupil is important. An effective diameter takes into account two main concepts:

• Eyesight
• Personal hunting style

For instance, if you have bad eyesight and prefer hunting in low light conditions, you should choose a large objective lens or a large exit pupil. 

I have a buying guide on low light scopes if you’re interested in some options.

Conversely, if you have great eyesight and prefer to hunt in brighter areas, choosing a small objective lens or a smaller exit pupil would be best.

In my case, it’s the latter.

I prefer hunting in the daytime and my eyesight isn’t terrible, so a smaller exit pupil is best for me.

What Is the Function of a Large Exit Pupil?

I made the mistake of assuming that a larger exit pupil diameter is better than a smaller exit pupil and is directly proportional to shooting better.

This would be the case if you have low light conditions but a large exit pupil doesn’t work as well in very bright conditions, like on a sunny day.

Take the human eye, for