Optics Terminology

Term Definition
Adjustable Objective (also referred to as parallax adjustment) Allows you to focus at distances over 100 yards and adjust for parallax. Parallax occurs when viewing distant targets and the reticle appears to shift or move. Most scopes without adjustable objectives are factory set at an optimum distance of 100 yards.
Elevation Vertical (up or down) adjustment of the scope.
Eye Relief The comfortable distance that a scope can be held from the eye and still allow the shooter to see the entire image. The more generous the eye relief, the better it allows the shooter to acquire the target more quickly, which is a must for running shots. 3"-4" is a good number that will fit most hunters.
Field of View (FOV at 100 yards) Means how wide of an area (in ft) you can view at 100 yards. A higher number indicates a wider area, while a smaller number indicates a narrower area.
Magnification (Power) Expresses the magnification as a factor compared to the naked eye (the first set of numbers, ex. 3-9x40 has a magnification range from 3x to 9x).
  • Low power riflescopes are ideal at close range and for shooting moving targets.
  • Medium power riflescopes are ideal for hunting big game targets at medium range.
  • High power riflescopes are ideal for shooting, when the target is motionless and for varmints and other small game.
Minute of Angle (MOA) Term used to designate variances on a target at 100 yards, most commonly used to describe the adjustment on a scope. If a scope's adjustments are listed at x MOA, then for every click of the adjustment knob, the bullet's point of impact will move x" at 100 yards.
Objective Expresses the diameter of the objective lens in millimeters (the last number, ex. 3-9x40 has a 40mm objective lens). Large lenses offer a wider field of view and let in more light, which makes your image clearer - especially in low light conditions.
Ocular Lens Is the lens closest to the eye.
Optical Coatings A thin chemical film used to coat the surface of the glass minimizing reflection. It reduces light loss and glare, increasing light transmission and resulting in brighter, clearer images.
  • Fully Coated » A single-coating on all air-to-glass surfaces.
  • Multi-coated » All internal lenses are single-coated and the air-to-glass lenses are multi-coated.
  • Fully Multi-coated » The brightest and most expensive option, all lenses are treated with multiple layers of coating. Resolution. Sharpness.
Reticle The aiming point within the scope - commonly called "crosshairs" due to the standard arrangement being two thin wires that cross.
30mm Tube The larger tube diameter gives added strength and rigidity due to the greater cross sectional area and larger rings and mounts. It also allows for an increased range of adjustment for windage and elevation.
Windage Horizontal (left or right) adjustment of the scope.
Zero The Mechanical Zero can be found by use of optical/mechanical tools like a bore scope or collimator. True Zero varies for each type of load used and must be found experimentally. Scopes are first mounted to Mechanical Zero, and then test fired and adjusted to a true zero.