Diopter Sight
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A diopter sight is an aperture sight component used to assist the aiming of ranged weapons, mainly firearms, airguns, and crossbows. 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. A diopter must be paired with a complementing front sight element to obtain a usable sighting line. 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. High end diopters typically accept accessories to aid the shooter's ability to see the target clearly. Diopters are found in particular as rear sight elements on
rifle A rifle is a long gun, long-barreled firearm designed for accurate shooting and higher stopping power, with a gun barrel, barrel that has a helical or spiralling pattern of grooves (rifling) cut into the bore wall. In keeping with their focus o ...
s. Diopter and globe sighting setups are commonly used in ISSF rifle shooting events.


Diopter rear sight

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. Through this small hole the shooter can view the front sight component(s) and the intended target. The typical occluder used in target shooting diopters is a disc of about in diameter with a small hole in the middle. The small diopter viewing opening ensures the shooter's eye is very precisely and consistently centered behind the diopter sight. The diopter sight is easy to use and usually allows for very accurate aiming, because a relative long sighting line can be used. A long sighting line helps to reduce eventual angle errors and will, in case the sight has an incremental adjustment mechanism, adjust in smaller increments when compared to a further identical shorter sighting line.


Principle

The "diopter sight effect" is achieved when looking through an aperture opening of approximately 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 The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...
is increased so that both the sights and
shooting target Shooting targets are objects in various forms and shapes that are used for pistol, rifle, shotgun and other shooting sports, as well as in darts, target archery, crossbow shooting and other non-firearm related sports. The center is often call ...
will appear sharp at the same time which further simplifies the aiming process. The
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
distance of a diopter sight is effectively adjusted to be the same as the sight distance. For example, with a distance of between the front and rear sight, the sighting system is effectively parallax adjusted to a distance of 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 at corresponds to an impact change of at . A rear sight with a larger aperture than 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.


Modern target sight setups

Typical modern target shooting diopters offer windage and elevation correction in increments of at (which equals 0.02 to 0.04  mrad or ≈ 0.069 to 0.172
MOA Moa are extinct giant flightless birds native to New Zealand. Moa or MOA may also refer to: Arts and media * Metal Open Air, a Brazilian heavy metal festival * MOA Museum of Art in Japan * The Moas, New Zealand film awards People * Moa ...
). Some ISSF (Olympic) shooting events require this precision level for sight adjustment since the score of the top competitors (last) shots series is expressed in tenths of scoring ring points. High-end target shooting diopters normally accept accessories like: *Rubber eye shields to restrain glare. *Anti-glare tubes. *Adjustable diopter aperture
iris diaphragm In optics, a diaphragm is a thin opaque structure with an opening (aperture) at its center. The role of the diaphragm is to ''stop'' the passage of light, except for the light passing through the ''aperture''. Thus it is also called a stop (an a ...
s (typically offering between diameter apertures) to increase the
depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...
and reduce reflections. *Optical filter systems to increase the
contrast sensitivity Contrast is the difference in luminance or color that makes an object (or its representation in an image or display) visible against a background of different luminance or color. The human visual system is more sensitive to contrast than to absolu ...
for the aiming eye. *Semi-transparent occluders for the non-aiming eye to ensure optimal sighting conditions for match shooters.


Complementing front sight element

The complementing front sight element may be a simple (hooded) bead or post for service arms, but is for target shooting more often a globe type sight, which consists of a hollow
cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...
with a threaded cap, which allows interchangeable differently shaped front sight elements to be used. Most common are posts of varying widths and heights or rings or holes of varying diameter — these can be chosen by the shooter for the best fit to the target being used. 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. High end target front sight tunnels normally also accept accessories like adjustable aperture and optical systems to ensure optimal sighting conditions for match shooters. Some high end target sight line manufacturers also offer front sights with integrated aperture mechanisms.


Diopter and globe sighting line principle

The use of round rear and front sighting elements for aiming at round targets, like used in ISSF match shooting, takes advantage of the natural ability of the eye and brain to easily align
concentric In geometry, two or more objects are said to be ''concentric'' when they share the same center. Any pair of (possibly unalike) objects with well-defined centers can be concentric, including circles, spheres, regular polygons, regular polyh ...
circles (circles all having a common centre). For optimal aiming and comfort the shooter should focus the aiming eye on the front sighting element. To avoid eye fatigue and improve balance the non-aiming eye should be kept open. The non-aiming eye can be blocked from seeing distractions by mounting a semi-transparent occluder to the diopter. For maximum precision, there should still be a significant area of white visible around the bullseye and between the front and rear sight ring (if a front ring is being used). Since the best key to determining center is the amount of light passing through the apertures, a narrow, dim ring of light can actually be more difficult to work with than a larger, brighter ring. 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.


Gallery

File:Diopter rear sight.jpg, Diopter rear sight on a Feinwerkbau 300S 10 metre target air rifle. File:2015 Department Of Defense Warrior Games 150612-A-ZO287-020.jpg, An adjustable aperture system () and occluder mounted on a
10 metre air rifle 10 meter air rifle is an International Shooting Sports Federation (ISSF) shooting event, shot at a bullseye target over a distance of using a calibre air rifle with a maximum weight of . It is one of the ISSF sports governed in shooting spor ...
diopter. Image:Hattie Johnson.jpg, An adjustable diopter aperture and optical filter system as used by Olympic 10 metre air rifle contestant
Hattie Johnson Hattie Johnson (née Ponti; born September 18, 1981) is a former Olympic athlete. The Athol, Idaho, resident competed in the Women's 10-metre Air Rifle at the 2004 Summer Olympics in Athens, Greece, finishing in 14th place. At the 2003 Pan A ...
. Image:Hattie Johnson 2.jpg, Taking aim through a diopter and globe sighting line. File:2014 Warrior Games Shooting Competition 141004-A-GC266-014.jpg, Competitor taking aim through a diopter and globe sighting line on a
Feinwerkbau Feinwerkbau , often abbreviated FWB, is a German manufacturer of firearms and air guns aimed mainly at competitive ISSF shooting events, including some contested at the Olympic Games as governed by the International Shooting Sport Federation (ISSF ...
P700 PCP target air rifle at the 2014
Warrior Games The Warrior Games is a multi-sport event for wounded, injured or ill service personnel and veterans organized by the United States Department of Defense. History 2010–2014 The Warrior Games have taken place annually since 2010 except ...
. File:Diana_diopter_sight.jpg, Diana diopter with anti-glare tube. File:Kongsberg_Arms_Factory_M-51_diopter_sight.jpg, Kongsberg Arms Factory M-51 diopter. File:Walther_diopter_sight.jpg, Walther diopter sight with mounted rubber eye shield to restrain glare File:Weihrauch_diopter_sight.jpg, Weihrauch diopter sight


References


External links


Head Position, Diopter Interval and Lint by Heinz Reinkemeier, UIT Journal at www.issf-sports.org
{{DEFAULTSORT:Diopter Sight Firearm components Firearm sights