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

Exit Pupil Scope

Do the light conditions mess with your eyes when you go hunting?

Don’t want to perceive dark images or dim objects anymore?

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?

Try adjusting the scope’s magnification to full, then directing 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 disc is the exit pupil.

What Is Exit Pupil on a Scope

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 rays of light 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 we 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.

The effect of the rays wouldn’t be the same for rays that are located off-axis. To solve this, we can 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 we 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.

Now let’s say that we’re looking at a 4 x 32 mm rifle scope, and we want to measure the exit pupil 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. 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 we find ourselves in bright light, the human pupil shrinks so that we can see a clearer image. When we find ourselves in low light conditions, the human 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 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, while 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. (We 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.

What Is the Function of a Large Exit Pupil?

A common mistake that’s often made is 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 (i.e., for night-time use), but a large exit pupil doesn’t work as well in very bright conditions (i.e., on a bright sunny day).

Take the human eye, for instance. In typical daytime, your eye’s pupil can be as small as 2 mm in diameter because of the bright light present.

When you use a large exit pupil, over half the light that goes through the scope falls on your iris rather than your pupil. The iris is the part of the human eye that controls how much light enters the eye.

  • It does this by changing the size of the human eye (more specifically, the pupil)
  • Your eye’s pupil can be larger (dilated) or smaller (constricted)

Since a large exit pupil allows more light to pass through, it forms a brighter image in low light conditions or during the night.

However, the maximum pupil size is 10 mm, which is based on the diameter of light that the human eye pupil can see.

Pros
  • Ensures that you get to see a clear image when it's dark/at night
  • Can ensure flexibility in regards to eye relief
Cons
  • Doesn't work as well in a bright area

What Is the Function of a Small Exit Pupil?

Conversely, a smaller exit pupil works best during the daytime.

In brightly illuminated light conditions,  the small exit pupil would help you see more focused light.

This means that a higher magnification is present.

This exit pupil wouldn’t work as well in nighttime conditions because you wouldn’t be able to see the image as clearly in the dark.

Pros
  • Gives you a clearer image in bright conditions/during the daytime
Cons
  • Doesn't work well in low light conditions/at night

What About the Binoculars Exit Pupil?

Did you know that the exit pupil is a concept that’s also present in various optical instruments?

If you’ve heard of these instruments before, the first thing to pop into your mind might be an astronomical telescope.

However, you can also find optical instruments in fields beyond astronomical observation.

If you search “binoculars exit pupil,” “typical daytime eye pupil,” or “virtual aperture,” they’d all yield results regarding optics.

In the context of binoculars, an exit pupil is the diameter of an image that is shown outside of the eyepiece.

Just like rifle scopes with a large exit pupil, binoculars with large exit pupils work best in low light because it produces a clearer image.

The formula for the size of the pupils in binoculars also goes as follows:

  • (Diameter of the lens) / (magnification of the binoculars)

If you have 7 x 50 binoculars, the diameter of your lens would be 50 mm, and the magnification would be 7x. This means that the diameter of the exit pupil calculated is 7 mm.

Simply put:

  • The larger the lens of your binoculars, the larger the diameter of the exit pupil.
  • The larger the magnifying power of your binoculars, the smaller the pupil diameter.

Determining the best exit pupil for binoculars means that you’d have to consider which light conditions you’d use them for.

People often prefer using 4 mm binoculars for the daytime and 5mm (or more) binoculars for the night.

However, some say that 4 mm binoculars can work well for the day/twilight/night.

Take note that the maximum size of a binoculars’ exit pupil differs, as multiple GIANT binoculars have exit pupils that are smaller than the ideal size.

Regardless, these binoculars are still able to produce clear images.

TRIVIA: If the human pupil is about the same size as the exit pupil size in binoculars, the images would be as clear as if you were looking at them with the human eye!

Choosing the best binoculars for you would entail determining the preferred size of your binoculars, the exit pupil’s maximum size or pupil diameter, and how much emergent light is present.

Conclusion

When choosing a good exit pupil for a rifle scope (or even for binoculars), you have to consider the best exit pupil diameter for you.

You can do so by taking note of:

  • Its image quality in certain lighting conditions
  • The best objective lens size
  • Your personal limitations

Taking all of these into account will help you ensure that you can use your rifle scope to its maximum capability!

FINAL TIP: Now that you know more about scopes, you may be interested in our buying guide on the top options for scopes under $500.

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