There are countless optics companies on the market, ranging from newcomers to brands that have spent decades building a reputation for durable, high-quality rifle scopes. Ask any shooter, and they likely have a go-to brand—much like the age-old debate between Chevy, Dodge, and Ford. However, the fundamental science behind rifle scopes remains consistent across manufacturers, just as combustion engines operate on the same principles despite brand-specific refinements. In this article, we’ll break down the science behind rifle scopes and explore common terminology to help you better understand how they work.

Parts of a Scope

1. Objective Lens
2. Maintube
3. Side Focus/Parallax Adjustment Dial
4. Windage Adjustment Turret
5. Elevation Adjustment Turret
6. Magnification Selector
7. Diopter Adjustment
8. Ocular Lens

Common Scope Terms

Field of View (FOV)

Refers to the width of the area visible through a rifle scope at a specific distance, typically measured in feet at 100 yards. A lower magnification provides a wider FOV, allowing for better situational awareness, while higher magnification narrows the FOV, focusing more on the target but reducing peripheral visibility.

Magnification

Refers to how much a rifle scope enlarges the image of a target compared to the naked eye. It is represented by a number, such as 3x or 10x, indicating that the target appears three or ten times closer than it would without the magnification of the scope. Variable power scopes, like a 3-9x, allow users to adjust magnification within this range for versatility in different shooting situations.

Eye Relief

This measurement indicates the distance between the eye and the optic where a totally clear image is obtained. This measurement can be important on heavy recoiling firearms.

Eye Box

The eye box is measured in conjunction with the eye relief and is the “range” within which you are able to maintain a full/clear sight picture.

Lens Coatings

The lenses used to manufacture optics are usually manufactured from glass, these lenses are then treated with proprietary coatings to serve a variety of purposes from scratch resistance to light transfer and clarity.

Reticle

The reticle is a general term for the aiming reference located inside the scope.

Sub Tension

When utilizing a reticle with multiple lines, circles, dots, etc. these features have a measurable value (example MIL or MOA). This value is the sub tension. Think of it as a measuring tape within your reticle.

First Focal Plane (FFP)

Also known as Front Focal Plane, and is when the reticle is positioned in front of the magnification lenses, meaning it changes size relative to the target as magnification is adjusted. This ensures that sub tensions remain accurate at all magnification levels, making FFP scopes ideal for long-range shooting and dynamic target engagement.

Second Focal Plane (SFP)

Also known as Rear Focal Plane, and is when the reticle remains a constant size regardless of magnification because it is positioned behind the magnification lenses. While this makes the reticle easier to see at lower magnifications, holdover and windage markings are only accurate at one specific magnification level, typically the highest setting.

Optical Triangle

Optics are governed by the laws of physics called the Optical Triangle. An optics prescription must find balance between the following properties – Magnification, Eye Relief, and Field of View. If you adjust one side of the triangle it will naturally affect the others. For example, if you increase the magnification of an optic, the field of view and eye relief will decrease.

Exit Pupil

Is the diameter measurement of the beam of light that travels through a scopes eyepiece that contains the image to the eye. This corresponds with the eye pupil dilation and its ability to observe light. To achieve the best optical clarity, colour contrast, and light transmission in your scope you want the exit pupil to be the same size, if not larger, than the diameter of the eye pupil at the given external light conditions or time of day. This can be calculated and adjusted based on the scope’s objective lens size and the magnification the scope is on.

How Light Travels Through a Rifle Scope

A scope does not gather light, rather it transfers light from the objective lens through a series of lenses in the erector system to your eye at the ocular lens. As light passes through each lens surface it loses a certain level of properties. This is why it is impossible to have a rifle scope with 100% light transmission. All optics can transfer light, but what is most important is the management of that light within the optic. There are several properties that affect the management of light – efficiency of the lenses, the reduction of glare, and the quality of the lens coatings that are applied to the glass.

How to Calculate Exit Pupil

As light is scarce at twilight hours of the day exit pupil becomes more important. During these low light conditions, on average, the human eye pupil is dilated between 4-5mm. This measurement is important in that maintaining an exit pupil with this range will allow the shooter to easily see their target within the scope during these low light conditions.

Calculating exit pupil is a simple formula that anyone can do by taking the diameter of the objective lens and dividing it by the magnification that the scope is set at.

Example: if a hunter is using a 4.5-14x40mm scope and sets the magnification to 4.5x, the corresponding exit pupil would be 8.88mm, which is more than enough exit pupil. However, if the magnification was set to 14x this would reduce the exit pupil size to 2.86mm and would make it more difficult to acquire a manageable target in lower light. To combat this, the hunter can back the magnification off to 10x which would present an exit pupil size of 4mm, which would be ideal to obtain the best sight picture in low light conditions.

Scope manufacturers take this formula into consideration when designing their scopes magnification ranges with the corresponding objective lens diameter. This is when the objective diameter shows its importance. It is necessary that as you increase magnification to also increase your objective diameter to maintain a good exit pupil.

It is important to note that a larger main tube on a scope does NOTHING for light transmission and is a common myth out there. Larger main tubes offer more turret adjustment internally and also allow more room for side focus or illumination systems. A larger objective lens may provide more light based on the exit pupil formula.

Understanding the science behind optics, rifle scopes specifically, is essential for making informed decisions when selecting an optic for your needs. Each component plays a crucial role in the overall performance of the optic. By grasping these fundamental concepts, shooters can better evaluate different scopes and make adjustments based on their specific shooting preferences. At the end of the day, the right scope isn’t just about brand preference, it’s about finding the right balance of features to enhance clarity, visibility, light transmission, and reliability in the field.

This information has been brought to you in thanks from Korth Group Ltd. - Outdoor Education.

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For more information check out Episode 42: Optics 101 & Scope Mountain Basics with Adam Patternson on the Harvest Your Own podcast here.