How does Exit Pupil on Rifle Scope Impact Performance?
One major feature of your rifle’s scope is the exit pupil which determines the power of your scope. It represents the light-based circular image that the scope presents to your eye. The exit pupil determines how much light passes through the scope to the human eye, what you see, and how clearly you can see objects.
The larger the size of the exit pupil, the brighter your view will appear. This is because the majority of your eye will be covered in light.
In this blog, we will examine the role and importance of the exit pupil in hunting and shooting. We will also review how the exit pupil can have an impact on your shooting skill and accuracy, during day and night conditions.
What Is Exit Pupil?
Turn the scope’s magnification to full and point the riflescope towards a bright wall or a patch of clear blue sky. Look through the eyepiece from a distance of about 10 inches. You will see a bright circular disc of light in the center of the field. That disc is the exit pupil.
The scope’s exit pupil represents the diameter of the light beam that leaves the eyepiece when you look through the lens. It is a small, virtual aperture that is always smaller than the eyepiece and usually measured in millimeters.
The size of the exit aperture matters. A large diameter exit pupil is considered more useful under low light conditions or when shooting at night because it allows light from a wider area to pass through, forming a brighter image. However, the maximum size of the exit aperture is no more than 10 mm at most. This is because the human eye can only see a diameter of light between 5 mm and 9 mm.
Understanding the Exit Pupil Function
You must have noticed that when you step into light from a dark room, you have to shade your eyes to protect them from the brightness. This is the case because when we go into a darker room, the pupils in our eyes expand allowing us to see more clearly. When we go into a brightly lit room, the pupils shrink and let a smaller diameter of light to pass through while still forming a clear image.
The same type of technology works in rifle scopes. During the daytime, a scope with a smaller exit pupil operates fine. But that scope does not work as well as another scope with a bigger exit pupil would at night.
The rays of light coming from a distant object, or your target, must pass through the virtual aperture and exit the scope before it can be seen in the human eye. The diameter of the exit pupil determines the amount of light and image that you can see.
A smaller exit pupil allows you to see more focused light. This is why higher magnification almost always comes with a lower exit pupil.
Riflescopes that have a larger diameter of the exit pupil allow light to pass in a bigger circular disc. This is particularly useful in dark, cloudy weather or when you are shooting at night. The bigger exit pupil size allows you to see more clearly and work even in dusky weather.
Is A Large Exit Pupil Better?
A large exit pupil diameter on a rifle scope can be better under some circumstances. It can be useful if you have to hunt during low light conditions. However, it can also limit the amount of light you see during a bright sunny day.
During the daytime, the pupil size of the human eye can be as small as 2 millimeters in diameter because of the bright sunlight around you. When you look through a rifle scope with a big exit pupil diameter, part of the rays of light coming through the scope will fall on the iris of the eye instead of falling on the pupil. The light that falls on the iris does not get reflected inside the eye and does not contribute to the brightness of the image you see.
Consider the photos shown above. When your pupil diameter shrinks to the size of 2 mm and you look through a rifle scope with an exit pupil of 7 mm, most of the light will fall outside your pupil and the brightness of the image will be lost.
However, if you are hunting or shooting at dusk, your eye pupil can become dilated to 7 mm. When you see through the same scope in the evening, the image will be brighter and clearer.
In short, a large exit pupil is pretty much as bad as an exit pupil that’s too small when it comes to getting the best sight image possible. An advantage of a large exit pupil is though that you have more freedom to position your eye on the optical axis as you will still see a large sight image.
It also can give you more flexibility in positioning your scope with regards to eye relief. Eye relief is the distance between your eye and the ocular lens. That’s the rear lens of your scope and a larger exit pupil diameter can allow you to move your scope around a little more..
So remember the simple rule. Larger exit pupil is good in the evening, but no use during daytime shooting.
How is the Exit Pupil Calculated?
You can find out the size of the exit pupil on your scope based on the magnification and diameter of the objective lens. It is calculated as the diameter of the objective lens divided by the magnification power.
For example, suppose that your scope’s objective lens diameter is 42 mm and it goes up to a magnification of 8x. The scope’s reading would be given as, 8×42.
The size of the exit pupil would be calculated as 42 ÷ 8 = 5.3. This means the exit pupil at maximum magnification on this scope would be 5.3mm in diameter.
A scope with a higher magnification will give a smaller exit pupil. Suppose that your rifle scope can go up to a magnification of 10x and has an objective lens diameter of 42 mm. The exit pupil would be calculated as 42 ÷ 10 = 4.2. So the diameter of our exit pupil, in this case, is 4.2mm. This makes the scope better for daytime shooting, but it may not be ideal for shooting in lower light conditions when the eye pupil is growing larger.
Now suppose that you get a scope with a bigger sized objective lens. Suppose that the objective diameter of your lens is 56 mm. At 8x magnification you will get 56 ÷ 8 = 7, giving you an exit pupil of 7mm which is excellent for shooting even at night. Even if it had a higher magnification of 10x, your exit pupil would be 56 ÷ 10 = 5.6, which isn’t bad.
As you can see, the exit pupil diameter depends on the objective diameter and the current zoom setting. A variable scope will therefore have a variable exit pupil size that will change when you change the magnification.
Does the Exit Pupil Impact Light Transmission?
The size of the exit pupil impacts the amount of light that transmits through the scope. A bigger exit pupil means that a wider disc of light is passing through the scope which can make images clearer during low light conditions.
One important consideration here is the brightness of the image. A 56mm lens that is zoomed out for 8x magnification would provide the same level of brightness as a 42mm lens zoomed out to a 6x magnification.
However, just because more light is passing through the scope does not mean that you will see all of it. As discussed previously, if your pupils are dilated, you will be able to see more of the image. If they shrink because you are standing in a bright area, you won’t see all the light that transmits through the scope’s exit pupil.
Remember the normal aperture of the human eye pupil can go from 2 mm to 9 mm based on the lighting condition.
How Does The Exit Pupil Impact Your Shooting?
An exit pupil that is smaller than the pupil diameter of your eyes will allow you to shoot with ease during the day but negatively affect your shooting at night. An exit pupil that is bigger than the eye pupil diameter of the human eye will lose some of the imagery around the edges.
In ideal circumstances, the diameter of your scope’s exit pupil should precisely match the size of the pupil diameter of your own eye. When this happens, it allows the highest degree of brightness and the biggest size of the target image to pass to your eyes.
Or, to put it in another way, when these two diameters differ, the light transmission ability of the lens is reduced. This makes it more difficult to identify details of the target and have a negative impact on your shooting skills.
Knowing the best magnification settings and lens sizes for all your scopes will always give you the best advantage in shooting. In the end, choosing the correct size of the objective lens comes down to your eyesight and hunting style.
If your eyesight is poor and you choose to hunt in low light conditions, a larger objective lens can make up for the difference and help compensate for the shortcomings. If you generally hunt during bright conditions, have fairly good eyesight or need good cheek and weld positioning before your shoot, a larger objective lens might hinder your shooting performance.
Do you have more questions or something to add? Please leave a comment below.