Diopter sight

Diopters function to precisely align the shooter's eye with the front sight and the target, while also producing beneficial optical effects for accurate aiming.

Diopter sights used for modern target shooting allow for very fine windage and elevation adjustments, moving the impact point on the order of less than five millimeters at a range of 100 metres.

The diopter is in principle a vertically and horizontally (elevation and windage) adjustable occluder with a small hole (aperture), and is placed close in front of the shooter's aiming eye.

The typical occluder used in target shooting diopters is a disc of about 25 mm (0.98 in) in diameter with a small hole in the middle.

Also, the depth of field is increased so that both the sights and shooting target will appear sharp at the same time which further simplifies the aiming process.

Typical modern target shooting diopters offer windage and elevation correction in increments of 2 to 4 mm (0.079 to 0.157 in) at 100 m (109.36 yd) (which equals 0.02 to 0.04 mrad or ≈ 0.069 to 0.172 MOA).

Tinted transparent plastic insert elements may also be used, with a hole in the middle; these work the same way as an opaque ring, but provide a less obstructed view of the target.

The precise sizes of the employed components are quite subjective, and depend on both shooter preference and ambient lighting, which is why target rifles come with easily replaceable front sight inserts, and adjustable aperture mechanisms.

Target shooting diopter of a 10 metre air rifle with a mounted semi-transparent occluder for the non-aiming eye
Rear, rotating diopter drum sight of a SIG SG 550 service rifle . The viewing aperture above the "3" (denoting the 300 m setting) can be seen.
Civilian AR-15 target sights have an aperture between 1 and 1.15 mm (0.039 and 0.045 in). The aperture on AR-15 military sights have a day aperture of approximately 1.78 mm (0.070 in), and the M16A2 also a night setting with a larger 5.08 mm (0.200 in), and as such the military sight is not strictly a diopter sight in either setting.
The concentric circle alignment principle is used for precise aiming at round targets