In this article, you will learn:
- Types of optics
- Basic terms in optics
- Basic definitions of optics
Understanding the terms used when describing the specifications and features of the different types of optics made for your firearm will help you decide which optic is best for your needs.
There are many words related to riflescopes, red dot and reflex sights that aren’t commonly used in everyday language but are incredibly important when it comes to describing the features of the optic. Here we describe the basic terms and definitions of optics.
Types of Optics
There are two types of optics you can mount on your firearm—magnified and non-magnified. Red dot tube, reflex, prismatic, holographic, digital and traditional scopes all fall under either of these categories.
These types of scopes are magnified:
- Prismatic (can also be 1x magnification)
- Traditional riflescopes
Magnification is the process of enlarging the appearance of an object through magnified lenses. For example, a 4x riflescope enlarges the image 4 times the size as seen by the normal, unaided eye. Riflescope magnification can be fixed or variable, ranging from 1x up to 40x and over. The magnification of the riflescope is designated by the first number in its optical configuration. For example, 4x32 for fixed and 3-9x40 for variable.
Prismatic scopes consist of various lenses and a prismatic lens set. They provide two benefits—no moving parts in the internal lens structure provide better durability and a higher probability of maintaining zero and prisms provide a folded focal length, reducing the overall length of the housing.
Digital Night Vision
Digital scopes use a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) and a microdisplay. Light projected onto the CCD or CMOS array from the objective lens is converted to an electronic signal. This signal is then processed and sent to the microdisplay to be viewed by the user. Digital night vision units require the addition of artificial light to create bright images but can be used in daylight conditions.
What is a Riflescope?
A riflescope is a telescopic sight that enlarges the image of what you’re looking at to help aim at a target and shoot accurately.
- Red dot tube sight
- Reflex open sight
- Holographic (HWS)
What is a Reflex Sight?
Reflex sights fall into two categories—open and tube sights. Open sights are generally referred to as reflex sights while tube sights are referred to as red dots.
Based on a reflector system, a reflex sight utilizes a reflective glass lens to project an illuminated image superimposed on the field of view. A reflective glass lens is used to collimate light from a light emitting diode (LED) to serve as an aiming point while allowing the user to see the field of view simultaneously.
Holographic sights, most notably made by EOTech, use a laser transmission hologram to produce an illuminated reticle or dot. The hologram is illuminated via a laser diode instead of an LED like in red dot sights.
The objective lens is the lens in which light enters the riflescope and is sharply focused. The diameter of the lens is measured in millimeters and designated as the last number in the scope’s optical configuration, 4-16x44. The larger the objective lens, the more light gathers and the result is a brighter image with higher resolution, sharpness and detail. Larger objective lenses deliver better images in low light conditions, but also create a heavier and more costly riflescope.
Field of View
The field of view (FOV) is the observable image visible through the riflescope. Field of view is measured in angular (degrees) or a linear field. Linear field measurements are the width in feet (or meters) of the viewing area at 100 yards (or 100 meters.) The wider the field of view, the greater the area you will see in the image. A wide field of view is helpful for close shooting ranges and moving targets. For variable power magnifications, the increase in power will also decrease the field of view.
Eye relief is the distance between the eyepiece and where the eye sees the full field of view with no dark edges around the image. Long eye relief in riflescopes is important. Recoil from rifles can hit your forehead or eye, causing injury if the eye relief is not great enough. Heavy-recoil rifles need a minimum of 2.5 inches, but 3 inches or greater is preferable. The downside to long eye relief scopes is they generally have a smaller field of view. With variable power riflescopes, eye relief will change. As power increases, eye relief will decrease.
A reticle is a set, or series of fine lines created from thin metal wire, etched glass, collimated light, or a computer-generated image superimposed on a screen. At its simplest form, the crosshair is represented as intersecting lines in the shape of a cross. Reticles are used as an aiming reference, with the crosshair being a representation of the bullet’s point of impact. Reticles are also designed to be used to estimate range to target and quickly designate bullet drop.
Windage and Elevation Adjustments
Windage and elevation adjustments allow the reticle to be zeroed to the point of impact of the rifle. Elevation controls the vertical (up/down) adjustment of point of impact and allows for compensation of bullet drop. Windage controls the horizontal (left/right) adjustment of point of impact and allows for compensation of wind deflection.
Parallax is the visual movement of the reticle in relation to the target. This movement is visible when the user moves their head and the reticle appears to swim over the target. Parallax is caused by the reticle not focusing at the same distance as the target.
Measured in millimeters, the exit pupil is the beam of light formed by the objective lens that exits the eyepiece and enters the user’s eye. The larger the exit pupil, the brighter the image; however, it is only applicable if the eye’s pupil is large enough to accommodate it. Large exit pupils are advantageous when viewing in low light. Exit pupil is found by the diameter of the objective lens divided by the magnification.
Reticles may be located on the First Focal Plane (FFP) or Second Focal Plane (SFP) of variable magnification riflescopes. In FFP configuration, the reticle remains at a constant size compared to the target. In low magnification, FFP reticles will appear small but grow with the increase of magnification. Second focal plane reticles remain the same size to the user while the target size changes.
Diopter Adjustment or Focus Ring
Diopter is the optical power of a lens and is a reciprocal length of focal length. Since everyone’s eyes are different, diopter adjustment compensates for variances between users. In riflescopes, the image is already focused by the objective lens, focus lens, and erector lenses, but diopter adjustment affects how the user’s eye sees the reticle. Typically, riflescope diopter adjustment ranges from +3 to -3, 0 being nominal 20/20 vision.
Milliradian (Mils) and Minute of Angle (MOA)
What Does MOA Mean?
Mils and MOA are understood as the graduation of a riflescope’s windage and elevation adjustment.
MOA is short for minute of angle and is also a unit for angular measurement. MOA is a smaller, finer measurement than one mil. 1 MOA is equal to 1.047” at 100 yards but rounded to 1.” A riflescope with 0.25MOA (1/4MOA) click adjustment means that each click will move the point of impact 0.25” at 100 yards.
Mil is short for milliradian, a trigonometric unit for angular measurement. Mils are a finer, more precise measurement than degrees. A single mil is equal to 3.6” at 100 yards or 10cm at 100 meters. A riflescope with 0.1mrad click adjustment means that each click will move the point of impact 0.36” at 100 yards or 1cm at 100 meters.
What Does Boresighting Mean?
Boresighting is the preliminary alignment of the optic’s reticle (sight line) to the trajectory line of a rifle.