What is the Objective Lens of a Scope?

Rifle scopes provide shooters the ability to magnify a target and increase shot precision especially when shooting at longer range targets. When searching for a rifle scope many shooters will mistakenly overlook the size of the objective lens and ask the question, what is an objective lens of a scope?

The objective lens of a scope is the lens furthest from your eye. Aside from glass clarity, the lenses in a scope are the most important parts. The objective lens is larger than the ocular lens and its purpose is to transmit light to the ocular lens. 

A scope is made up of several parts which are all vital to its performance, but none may be as important as the objective lens. This article will explain everything you need to know about this essential piece and what its purpose actually is. 

Is Bigger Always Better?

When looking at rifle scopes you may notice that there are several numbers to look at. For example you may see a scope with 3-9x40 on the box.  The 3-9 is the magnification, and the 40 is the diameter of the objective lens in millimeters.

Objective lens diameters may be as small as 28mm to over 50mm. The wider the objective lens, the more light is allowed into the scope and transmitted by your eye. This allows for a larger exit pupil, improved image in lower light conditions and at higher magnification, and an increased field of view.

Although a larger objective lens sounds like a no brainer there are other possible downfalls that accompany them. A wider objective lens is housed by a larger objective bell making for a higher scope profile and possibly causing a less than ideal cheek weld. These scopes are also heavier and more expensive.

Aside from overall scope size and profile there are several other factors to consider that relate directly to the size of a scopes objective lens.

Importance of Exit Pupil

Now that you understand what the objective lens is and the purpose it serves, it is time to dig into the exit pupil and how it relates to the size of the objective lens. By definition the exit pupil is the amount of light that is being transmitted to your eye and it is the beam of light visible through the ocular lens 10 inches away from it.

The majority of people have a pupil of 5 to 7mm which means if your scope has an exit pupil greater than 7mm then it will deliver more light than your eye can actually use.

The exit pupil of a scope is calculated by dividing the objective lens diameter by the magnification. For example, a 3-9x40 scope would have a 13.3mm exit pupil at 3x magnification and a 4.4mm exit pupil at 9x magnification. This means at a lower magnification your eye will be provided with the greatest amount of light.

Any exit pupil that exceeds 7mm is delivering more light than your eye can use which allows for greater flexibility with regards to eye position. In hunting situations this means a shooter is able to mount their rifle quicker without having to align their eye perfectly with the optical axis.

When target shooting in lighted conditions a large exit pupil is not necessarily beneficial as there is plenty of time to line up your eye.

Exit pupil is a very important consideration especially in low light conditions. The more light that is transmitted to your eye, the more usable light you will have for shooting in low light conditions either early morning, late afternoon or on dark days.

Magnification Matters

A larger objective lens seems like the obvious choice especially for hunting applications if weight and scope profile are not limiting factors, but there is still more to consider. Scope magnification plays a large role in determining the correct objective lens size.

The objective lens diameter and magnification go hand in hand. A scope with a larger objective lens will get more magnification out of a high powered scope before the image starts to deteriorate. When using a high powered scope in low light conditions a larger objective lens gives you more usable light and a clearer image.

A scope with less magnification such as a 1-4x will not necessarily need a large objective lens. In fact, a smaller objective lens will give you plenty of light and increased efficiency with these low powered scopes.

The larger objective lens increases the amount of usable light and improves image quality at higher magnifications especially in low light conditions. In short, the more magnification a scope has, the better off you are with a larger objective lens.

What Size Is Best?

A larger objective lens is beneficial when using a scope with higher magnification, shooting in low light conditions, and when you would like a wider field of view. Although this is all true, your intended shooting application will be the deciding factor.

A close range rifle, for example, a .22LR with a low power 1-4x scope, a smaller objective lens will be more efficient and do exactly as you want. When considering a longer range rifle, for example, a .300WM paired with a higher power 4-12x scope, a larger objective lens is ideal as the image will be much more clear and crisp at these higher magnifications.

For a more versatile scope it would be best to choose a 36mm to 44mm. These are the most common sizes and allow for a great field of view and clarity of the intended target even in low light applications.

Everyone Is Different

Always consider the scope magnification, possible shot distances, weight, scope profile, rifle uses, and hunting or shooting conditions. It is best to try out different scopes with a variety of objective lenses first hand, because what may be the right choice for someone else isn’t always right for you.

Objective lens is the most important part of your scope aside from the actual glass quality. A larger objective lens will allow for more light to reach your eye but it certainly can’t fix bad glass, so opt for the best quality scope in your price range.

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