page_title: Diopter sight
section_title: Principle
hierarchical_section_title: Diopter sight / Diopter rear sight / Principle
caption_attribution_description: Looking down the rear iron sight of an M15A4 Carbine. English: Civilian AR-15 target sights have an aperture between 1 to 1.15&#160;mm (0.040-0.045"). The aperture on AR-15 military sights have a day aperture of approximately 1.8&#160;mm (0.070"), and also a night setting with a larger 5&#160;mm aperture (0.200"), and as such the military sight is not stricly a diopter sight. Norsk bokmål: Et dioptersikte på en Armalite AR-15. Sivile AR-15 matchsikter har lysåpning mellom 1 til 1.15 mm åpning (0.040-0.045 tommer). Baksiktet på militære utgaver har et dagsikte med lysåpning på ca. 1,8 mm (.070"), samt nattsikte med større åpning på 5 mm (0,200"), og de militære siktene er således strengt tatt ikke dioptersikter.
context_page_description: The diopter is an aperture sight component used to assist the aiming of devices, mainly firearms, airguns, and crossbows. It is found in particular as a rear sight element on rifles.
  To obtain a usable sighting line the diopter has to have a complementing front sight element.
  Diopter and globe sighting lines are commonly used in ISSF match rifle shooting events.
context_section_description: The "diopter sight effect" is achieved when looking through an aperture opening of approximately 1.2 mm (0.047 in) or less, and happens due to an optical phenomenon (edge effects) resulting in the light passing through being parallelized similar to how a collimated lens would. Because of this optical effect the front sight will also appear more steady, even though the shooter moves the head in a way such that the sighting eye moves sideways relative to the rear sight. 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.
  The parallax distance of a diopter sight is effectively adjusted to be the same as the sight distance. For example, with a distance of 1 m (1.1 yd) between the front and rear sight, the sighting system is effectively parallax adjusted to a distance of 1 m (1.1 yd) in front of the rear sight. However, since the diopter hole is small, the aiming eye will be relatively well centered, and the parallax error will be relatively small in practice. There will be almost no parallax error if the eye and front globe sight are near perfectly centered through the diopter. In comparison, the parallax error on aperture sights with larger openings can increase quickly if the eye is not well centered in the rear sight. A parallax error of just 1 mm (0.039 in) at 1 m (1.1 yd) corresponds to an impact change of 10 cm (3.9 in) at 100 m (109.4 yd).
  A rear sight with a larger aperture than 1.2 mm (0.047 in) is not strictly a diopter sight, but nonetheless is still often (incorrectly) referred to as such. With larger aperture sights the shooter must make a conscious effort to center the eye in the rear sight for precise aiming. A true diopter sight (aperture below 1.2 mm) however has the advantage that the shooter does not have to concentrate on eye and rear sight alignment for precision aiming, and therefore the sighting process is reduced to only aligning the front sight to the target.
  Aperture sights (both diopter and non-diopter) require being placed close to the aiming eye, while open sights have to be placed at least 30 cm away from the eye to in order to appear sharp.
caption: Civilian AR-15 target sights have an aperture between 1 to 1.15 mm (0.039 to 0.045 in). The aperture on AR-15 military sights have a day aperture of approximately 1.8 mm (0.071 in), and also a night setting with a larger 5 mm (0.20 in), and as such the military sight is not strictly a diopter sight in either setting.
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