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The Objective Lens is lens closest to the target or the lens at the front of the scope furthest from the operators eye.
Considerations when choosing an Objective Lens are:
- Lens Optic Quality
- Lens Optical Coatings
- Ambient Light
- Adjustable Objective (AO) Lens
- Exit Pupil
It is all about the light. Light is the reason we see things. Light waves in the visible spectrum reflect off a surface and are transmitted to your eye.
Therefore the amount of light transmitted to the eye determines how well we can see an object.
The objective lens diameter is a major factor in determining how much light is delivered to the eye. Most scopes are designed with small to medium objective lens diameters. Fine for most light conditions. For low light conditions it might be desirable to have a larger diameter objective lens.
If the objective lens is large then the objective tube or bell will be large. This can effect the scope clearance from the rifle. This could mean a requirement for non standard (higher) mounting rings, which usually means $$$ dollars.
More importantly, higher mounting rings may take your cheek off a good resting position on the stock. Accuracy can suffer as a result.
A large objective lens also has an effect on our next consideration.
Adding a scope with a large objective can add a fair bit of weight to your rifle and make it top heavy at that. This can make the rifle awkward and unwieldy. Some scopes come with Adjustable Objective(AO) lenses which tend to compound the problem
Lens Optic Quality
We have established that more light means a brighter image. If the glass the lens is made of has impurities and is poorly finished then the resulting image delivered to the eye will suffer.
Lens Optic Coatings
Optic coatings can mean internal light loss and glare are reduced. Ensure light transmission is uniform, leading to improved image sharpness and contrast. Some coatings are designed to interfere with wavelengths of light, eliminating their reflection.
Overall, coatings try to maximize the light entering the lens and enhance the delivery through to the eye. Coatings can vary from scope to scope and manufacturer.
Given that light is such an important consideration the ambient light available in the environment you intend to be operating in can be a major factor.
If you are in the open plains of central Australia on a bright Summer day your objective lens diameter will not need to be large. On the other hand if it is Winter time and close to dusk and you are hunting drop bears in Tasmania’s great South Western forests. Then you will likely need the biggest objective lens you can handle.
Adjustable Objective (AO) Lens
The adjustable objective lens (AO) is a feature that allows the shooter to fine-tune the focus of the reticle or crosshairs. This is especially useful when shooting at long ranges, where small changes in focus can make a big difference in the accuracy of the shot.
Adjustable Objective (AO) Lens
If you are shooting at range with a scope that has its reticle at the Second Focal Plane (SFP) then parallax error is likely to be a consideration and a scope with AO or parallax adjustment might fit the bill.
Exit pupil is covered in a an article we wrote here:
While we love certain things about certain brands, favor one for their optic quality. Another for extra features you may want or price.
My first scope was a Tasco with a 40mm objective lens bought in 1980, reasonable optics and price and it still does the job nicely today. (Yes, I still have it.)
At the end of the day it depends on what suits your needs in terms of task, environment and price. Do your research and you should achieve a good outcome.
The Objective Lens in Optic Terminology
When you see a rifle scope referred to in language or text it is conventionally given as “4 x 30″ for example.
The “4” refers to how much the target image has been enlarged i.e. an object seems four times closer.
The “30” indicates the objective lens’s diameter in millimeters (mm ).
So a 4 x 30 scope has a 4 times magnification and a 30 mm objective lens.
FAQ (Frequently Asked Questions)
What is the objective lens of a scope?
The objective lens is the lens closest to your objective or target, the one farthest from your eye. The objective lens gathers the visible light reflected off an object and transmits it via the ocular lens (eyepiece) to your eye.
Is a larger objective lens better?
The larger the objective lens is the more light it collects, resulting in a better, brighter image. This can be useful in low ambient light conditions and at higher magnifications.
What is objective lens vs ocular lens?
The objective lens is the lens or lens array closest to the object being viewed. The ocular lens is the lens or lens array closest to the viewers eye, also known as the eyepiece.