Optics basics

Red dot and holographic sights

A red dot sight projects an illuminated aiming point that the user sees superimposed on the target, usually without magnification. Examples in the catalog include the Aimpoint CompM4 and the Trijicon MRO. Holographic sights such as the EOTech EXPS3 use a laser-recorded reticle image instead of a reflected LED dot.

Fixed and variable magnification

Magnification is written as a number followed by x: a 4x optic makes the target appear four times closer. Fixed-power optics such as the ACOG TA31 trade flexibility for simplicity. A low-power variable optic (LPVO) like the Vortex Razor 1-6 covers a range starting at or near 1x.

Field of view, eye relief and parallax

Field of view is the width of the scene visible through the optic at a given distance; it narrows as magnification increases. Eye relief is the distance between the eye and the rear lens at which the full image is visible. Parallax is the apparent shift of the reticle relative to the target when the eye moves off the optical axis.

MOA versus MIL

Adjustments and reticles are measured in angular units. One minute of angle (MOA) is 1/60 of a degree, roughly 29 mm at 100 m. One milliradian (MIL) is a thousandth of a radian, 100 mm at 100 m. Neither is more precise in principle; what matters is that reticle and turrets use the same unit.

Some optics add a bullet drop compensator reticle with marks for typical distances. Zeroing is the process of aligning the sight with the point of impact at a chosen distance.