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  • Post last modified:August 13, 2023

In this quick informational guide, it's talk about the difference between 40mm and 50mm, and what does a bigger objective lens do as they're quite popular for modern riflescopes.

The scope's objective lens size is measured as a diameter, which controls how much light it can gather. The bigger the objective lens diameter the better the light transmission, which also results in better image quality in low light or cloudy conditions.

In addition, having high quality glass coating and the glass will enhance the image sharpness.

Besides light transmission performance, there are other considerations, which we will cover in this guide.

Learn more below:

40mm VS 50mm Core Differences

Under the naked eye, there is not much difference between a 40mm objective lens scope and a 50mm objective lens scope.

A 40mm lens with high-quality glass and coating can gather and transmit more light to the shooter's eye than a 50mm lens without good quality lens coating.

Exit Pupil Size Matters In Low Light

Rifle scope eye box illustration

An advantage of a 50mm Objective Lens Scope is that it gives a larger exit pupil than a 40mm, which will give greater flexibility for head movement and get on target fast without too much scope shadow getting in the way.

Under bright conditions, human pupil has between 2-3mm, and under low light condition, it's about 7mm.

The brightness of the image the shooter sees is depended on the exit pupil size of the riflescope at that particular magnification power.

Weight Consideration


A 50mm objective lens scope typically adds to the overall weight of the scope. For hunters who are planning on a long hike and also know how much magnification they really need for that perfect shot, a smaller objective lens might be the choice to go.

Generally speaking, a 50mm to 56mm objective lens scopes have the penalty to be paid for increased performance in the form of substantially increased weight and cost.

In addition, the large tapered objective lens diameter makes mounting a scope difficult on some rifle platforms, which requires elevated mount height for better scope clearance.

Lens Coating Matters

A rifle scope with high quality lens coating will allow more light to pass through the lens. A scope without good coating will reflect light and also internally, which creates lens flare and light bouncing off the lens.

When light is bouncing and reflecting in and out of the rifle scope, the image quality will be poor and sometimes appears blurry or smeared.

Scope Glass Quality Matters Too

rifle scope lens and aiming reticle

The quality of that glass can mean the difference between a great scope and a okay scope.

Typically, with scope glass, you generally get what you pay for. For the most part, the better the glass, the better resolution and the more expensive the scope model.

So how do you tell which ones are good?

There are 3 things to look at:

  • Clarity
  • Brightness
  • Color (No glass tint is highly preferred)

Most rifle scopes you see on the shelves today are made overseas. Japanese glass are the best out of all, and they supply these glasses to NightForce, Vortex, Primary Arms and more.

Scope Tube Diameter


A wider objective lens can provide the shooter with a brighter image assuming everything else is constant.

A typical common misconception in the scope world is – a wide objective lens combined with a large tube diameter gives the best image quality, which is false

A 56mm objective lens with a 34mm tube doesn't necessarily mean it's better if the glass quality is poor.

To case a point, we see the premium grade Vortex Razor HD LH with a smaller objective lens showing sharper and brighter image than an entry level Vortex Crossfire 2 Hog Hunter 3-12X56mm.

Scope tube size does allow additional adjustment clicks for a long range scope, learn more about it here.

Typical Scopes With 40mm Objective Lens Size





If you're buying a scope for long-range shooting as a beginner and not sure what's the best option before spending big money, please check out our guide on the best long range scopes under $1000

Rifle Scope Mounting Clearance & Cheekweld

Burris XTRII 5-25X50 SCR MIL MRAD FFP Scope
Notice how the scope almost touching the barrel?

A larger objective lens scope will need a taller scope mount for the following reasons:

  • The front of the scope doesn't touch the rifle barrel or handguard
  • Barrel harmonics can make contact with the scope during recoil if the scope sits too low, which can cause damage and accuracy issues
  • Might need a rifle chassis system with an adjustable stock cheek riser