Iron sights

The term iron sights refers to the open, unmagnified system used to assist the aiming of a variety of devices, usually those intended to launch projectiles, such as firearms, airguns, and crossbows; they are also used on many telescopes to help point the telescope at a desired target. Iron sights usually consist of some form of notch or aperture in the rear sight and a post, bead or ring in the front sight. Often, the rear sight is adjustable for windage and/or elevation, though in many military rifles, the front sight is also adjustable.

This article will concentrate on firearms sights; the principles described are equally applicable to any device which needs aiming. For the sake of brevity, the term gun will be used to indicate any device aimed by iron sights, the term shooter will be used for the operator of said device, and the term target will be the object at which the device is being aimed.

How iron sights work

Iron sights work by providing horizontal and vertical reference points that allow the shooter to align the gun parallel to the shooter's line of sight. Once the sights are aligned with each other, they are placed in correct relation to the target. This places the gun at a precise angle to the line of sight to the target. With appropriate compensation for range to the target, parallax between the iron sights and the gun's bore, and the trajectory of the projectile, a shot fired will hit the target.

A sight picture with focus on the front sight; the gray dot represents the target.

Since the eye is only capable of focusing on one plane, and the rear sight, front sight, and target are all in separate planes, only one of those three planes can be in focus. Which plane is in focus depends on the type of sight, and one of the challenges to a shooter is to keep the focus on the correct plane to allow for best sight alignment.

A tiny error in sight alignment can be multiplied hundreds or thousands of times by the time the projectile reaches the target; for example, with an Olympic-class air rifle shooter trying to hit the 10 ring, which is 1 mm in diameter, with a 4.5 mm diameter pellet at 10 meters, an error of 0.2 mm in sight alignment can mean a miss. At 1000 meters, that same 0.2 mm misalignment would be magnified 1500 times, giving an error of over 300 mm. (Calculations assume a 660 mm sight radius)

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Types of iron sights

Iron sights are broken into two basic categories that include most types. Open sights use a notch of some sort as the rear sight, while aperture sights use a circular hole. Wing and clay-shooting shotgun sights are called shotgun beads, or simply beads.

Open sights

A selection of open sights, and one aperture sight suitable for use with long eye relief: A) U-notch and post, B) Patridge, C) V-notch and post, D) express, E) U-notch and bead, F) V-notch and bead, G) trapezoid, H) ghost ring. The gray dot represents the target.

Open sights generally are used where the rear sight is at significant distance from the shooter's eye. They provide minimum occlusion of the shooter's view, but at the expense of precision. Open sights generally use either a square post or a bead on a post for a front sight. The post or bead is placed in the rear sight notch, and the target is placed above and centered on the aligned sights. From the shooter's point of view, there should be a noticeable space between each side of the front sight and the edges of the notch; the spaces are called light bars, and the brightness of the light bars provides the shooter feedback as to the alignment of the post in the notch. Vertical alignment is done by lining up the top of the front post with the top of the rear sight, or by placing the bead just above the bottom of the U or V-notch.

Patridge sights are the most common sights used for target pistol shooting, as most shooters find the vertical alignment is more precise than that of other types of open sight. Express sights are generally considered the fastest of the open sights, as the wide, shallow "V" obscures less of the shooter's vision, and the usually large front bead (similar to a shotgun sight) is easy to find in a hurry. The express sight is the only type of open sight where it is considered acceptable to focus on the target, and not the front sight. In cases where speed far outweighs accuracy (e.g. the shooter is being charged by a Cape Buffalo), then the front sight is used like a shotgun bead; the rear sight is ignored, and the bead is placed below the target. When more time is available, then the focus should be on the bead, which allows more precise placement of the bead in the "V" of the rear sight.

Aperture sights

Pictures taken under identical conditions through large (left) and small (right) diameter aperture sights, with camera focused on front sight.

Aperture sights range from the ghost ring sight, whose thin ring blurs to near invisibilty (hence ghost), to target aperture sights that use large disks with pinhole-sized apertures. In general, the thicker the ring, the more precise the sight, and the thinner the ring, the faster the sight.

The image to the right shows a shooter's eye view of the sight picture taken through large and small diameter apertures. The large diameter aperture provides a much brighter image of the target, and the ghosting of the rear ring is evident. The smaller aperture, while providing a much darker image of the target, provides a much greater depth of field (see pinhole camera for an explanation of this effect}, yielding a much sharper image of the target.

Ghost rings

A ghost ring sight

The ghost ring sight is the fastest type of aperture sight. It is fairly accurate, easy to use, and obscures the target less than nearly all other non-optical sights. Because of this, ghost ring sights are typically found on military, police, and self-defense firearms of all types—pistols, rifles, and shotguns. The ghost ring is a fairly recent innovation, and differs from traditional aperture sights in the extreme thinness of the rear ring, and the slightly thicker front sight. The thin ring minimizes the occlusion of the target, while the thicker front post makes it easy to find quickly.

Target aperture sights

A target rear sight with different front sight inserts: A) a ring with horizontal bars, which act as a level reference, B) a ring with a vertical post, C) a simple post, D) a transparent disk with a yellow tint. The gray dot represents the target.

Rear aperture of a BRNO target sight. Note large disk and small aperture, characteristic of a target sight, plus large adjusting knobs for elevation and windage.

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