A riflescope has two outward-facing lenses: an ocular lens and an objective lens. The ocular lens is the one you look into, and the objective lens is the one you look out of.
Typically, but not always, the objective lens is larger than the ocular lens. There are few ways you can compare one objective lens with another, one of which is the diameter.
This article is all about diameter and why it matters. Let’s dive right in.
What Exactly Is an Objective Lens Diameter?
The diameter of the objective lens is a measurement of how long a line is from one edge of the lens to the other when it has to pass through the center. Since the objective lens is circular, a larger diameter always means a larger circumference, so the measurement of the diameter is a quick, simple way to compare one objective lens with another.
The size of the objective lens is an important thing to know, understand, and consider when you are purchasing a scope for your rifle.
But why does it matter, and how will it affect your shooting experience when you choose one size objective over another? Should you base your decision on which scope to buy purely based on objective diameter? Let’s talk about it.
Why Is the Objective Lens Diameter Important for Riflescopes?
The size of the objective lens has a big impact on a few aspects of using your scope.
It’s not the end-all be-all of choosing the right scope, but it’s an important factor to consider.
The size of the objective matters more when you have more magnification, but it’s good to be aware of even if you’re looking at an LPVO.
1. Light Transmission
The most drastic effect that the size of the objective lens has on the image you see is light transmission.
The larger the objective lens, the more light can be let into the scope and the brighter the image will be.
However, there’s a ceiling to how much you’ll notice the increase in brightness, and that ceiling is caused by the size of the pupil in your eye.
The brighter your surroundings, the smaller your pupil will shrink so that it lets in less light and maintains an optimal (theoretically) level of brightness in the world around you.
In other words, once your objective lens diameter is large enough for the image through the scope to be as bright as the world seen through your naked eye, there is no additional brightness benefit to getting a larger objective lens.
So, you may ask, how do you know if your objective lens is big enough?
We can figure out how large your objective lens needs to be in different situations by calculating the exit pupil.
The exit pupil is a measurement of how much light is coming out the ocular lens and reaching your own pupil.
If the exit pupil is larger than your own pupil at the time, then the image in the scope will appear roughly as bright as what you see in your naked eye.
If, however, you end up in a situation where your pupil is larger than the exit pupil, then the image in the scope will be darker than what you can see normally.
This sort of thing happens most often as the sun starts to set and you’re still shooting, but it can happen anytime you’re shooting in darker conditions.
You calculate the exit pupil by dividing the diameter of the objective lens by the power you have the scope set to.
Say, for example, that you have a scope that is set at 10x magnification and you have a 50mm objective lens (pretty big).
This means you have a 5mm exit pupil. Since your own pupil may only grow to 5 or 6mm in low-light conditions, you won’t notice a darker image until it gets pretty dark.
The maximum size of a human pupil is right around 8mm, so if you can get your exit pupil to at least 4mm, you will not often run into situations where the image in the scope is too dark to shoot while you can still see well enough with your naked eye.
One of the problems comes with a variable magnification scope intended for long distance – like a 6-24x scope, for example.
A lot of those scopes come with a 50mm objective, but at 24x that only gives you an exit pupil slightly larger than 2mm, so you may notice the image get darker as you zoom in even if you’re in bright daylight.
Here’s another example that is more common: 3-9x scopes. A lot of 3-9x scopes come with a 32mm objective, and a lot come with a 40mm objective.
My recommendation would typically be to get the 40mm objective because the extra size will make a big difference in your low-light capabilities.
The size of the objective lens also can affect how sharp an image can be, because it can capture more detail when it’s larger.
However, the effect of the objective on sharpness is not nearly as dramatic as it is on brightness. It’s a lot like your eye – when you’re out in bright sunlight, even after your eyes have adjusted, you can’t see quite as much tiny detail as you can in optimal lighting.
That said, the darker your surroundings are, even after your eyes adjust you won’t be seeing as much detail simply because there isn’t enough light, but (as far as I understand it) that’s why your eyes see noise in really dark situations.
I would say that the sharpness aspect is only worth considering if you are intending to do long-range precision shooting and will be applying magnification of 20x or greater to do so.
3. Physical Size
Once you start talking about 50mm objectives and larger, and even 40mm objectives in some cases, you have to mount the scope higher up away from the rifle to avoid contact.
Usually all this takes is taller rings, but you need to make sure to purchase the correct rings to make it all fit the way it needs to.
This is one of the biggest reasons to not buy a larger objective than you’ll need. If you’re using a 3-9x scope, don’t bother with any objective larger than 40mm.
If you’re doing an LPVO that only goes up to 4x or 6x, you probably don’t need anything larger than a 24mm objective.
Also, the larger you have to mount the scope to accommodate the objective lens, the more it will affect your cheek weld and other aspects of your shooting stance.
Tips to Choose a Rifle Scope’s Objective Lens Diameter
Bigger Is Not Always Better
Going off of my last point, bigger is not always better. Bigger objectives add more weight to your set-up, can require you to buy taller scope rings, and even make you change aspects of your shooting stance.
Yes, you want to make sure your exit pupil is larger or at least not much smaller than your actual pupil, but as soon as you’ve got that done, stop going larger.
The disadvantages are real and there’s no reason to keep going bigger after you’ve passed the pupil threshold.
Do the Math
Just remember to divide the diameter of the objective by how much magnification you are applying. 32mm objective at 8x magnification gives you 4mm exit pupil.
24mm at 4x gives you an 8mm exit pupil. Consider the type of shooting you’ll be doing and in what conditions you’ll be doing it.
Run the calculation at both ends of the magnification range of the scope you’re considering and see if it will keep your exit pupil large enough for the level of brightness you’ll be shooting in.
Consider How You’ll Use It
If you only ever intend to shoot during the day, then the size of the objective won’t be as important because it’s less likely that your exit pupil will ever be in danger of being smaller than your actual pupil.
If, on the other hand, you’re a nighttime hunter, then you may want the biggest bell you can find and it may very well be worth dropping a lot more money on the scope that gives you the largest objective lens.
The Role of Coatings
When it comes to image brightness, clarity, and sharpness, manufacturers can get more mileage out of a smaller objective lens diameter by using chemical coatings, but the difference is typically subtle.
Yes, if you buy a $1,500 Leupold with a 32mm objective, you may get as good of an image as you do out of a $50 CVLife with a 40mm objective, but those are two extremes on opposite sides of the spectrum.
A Vortex with a 40mm objective will almost always be brighter than an Athlon with a 32mm objective, and vice versa. A Leupold and a Nightforce priced similarly are going to have much more noticeable differences based on objective diameter than on proprietary coating formula.
How Does Diameter Affect the Objective Lens?
So, as quick summary, the diameter of the objective lens will affect how much light is able to pass through the scope and eventually reach your eye. The most dramatic effect this will have is on the brightness of the image, but it will also affect the sharpness.
The size of the objective can also cause issues mounting and require you to use taller scope rings to prevent the objective lens from coming in contact with the rifle barrel or handguard.
Objective lens diameter means the same thing for binoculars as it does for riflescopes. You don’t have the added consideration of accommodating the size of the objective when mounting because binoculars typically don’t mount to tripods anywhere near the objectives.
The effects on image brightness and clarity are the same for binoculars as they are for riflescopes.
Sometimes. If the image you are seeing in your scope is darker than what you see with your naked eye, then getting a larger objective would make the image better. That said, so would reducing your magnification, so it may be worth practicing at a lower power than investing in a whole new scope.
There are a few reasons why your objective lens might be small in diameter. Number one is that the scope has low magnification. For example, a 24mm objective is common in LPVOs, but would be hilariously insufficient for a long-range scope. Another reason is that the scope was cheap and made by a company that isn’t interested in making a quality product.
I hope this article helped you understand a little bit more about objective lens diameter. It’s an important component of your riflescope and getting the right size of objective for the magnification and shooting conditions you’ll be working in can make a huge difference in the quality of your shoot.
It can also affect how enjoyable your experience is – it’s not much fun trying to squint through a low-quality image to see your target. If you noticed anything I got wrong in this article, feel free to let me know in the comments. I don’t claim to know everything about riflescopes, so having some experts weigh in is always valuable.