Editing Parallax (section)

==Weapon sights==
Parallax affects [[sight (device)|sighting devices]] of ranged weapons in many ways. On sights fitted on [[small arms]] and [[Bow and arrow|bows]], etc., the perpendicular distance between the sight and the weapon's launch axis (e.g. the [[bore axis]] of a gun)—generally referred to as "''sight height''"—can induce significant aiming errors when shooting at close range, particularly when shooting at small targets.<ref>{{cite web|url=http://www.dexadine.com/bexhelp/bexhelp23.htm|title=Ballistic Explorer Help|website=www.dexadine.com|url-status=live|archive-url=https://web.archive.org/web/20110928025148/http://www.dexadine.com/bexhelp/bexhelp23.htm|archive-date=2011-09-28}}</ref> This parallax error is compensated for (when needed) via calculations that also take in other variables such as [[bullet drop]], [[windage]], and the distance at which the target is expected to be.<ref>{{cite web|url=http://www.crossbowmen.com/index.htm.trajectory.html|title=Crossbows / Arrows & Bolts / Trajectory / Trajectories.|website=www.crossbowmen.com|url-status=live|archive-url=https://web.archive.org/web/20110708205657/http://www.crossbowmen.com/index.htm.trajectory.html|archive-date=2011-07-08}}</ref> Sight height can be used to advantage when "sighting in" rifles for field use. A typical hunting rifle (.222 with telescopic sights) sighted in at 75m will still be useful from {{cvt|50|to|200|m|yd}} without needing further adjustment.{{citation needed|date=June 2012}}

===Optical sights===
{{further|Telescopic sight#Parallax compensation}}
[[File:Telescopic Sight Parallax Animation.gif|thumb|Simple animation demonstrating the effects of parallax compensation in telescopic sights, as the eye moves relative to the sight.]]

In some [[reticle]]d [[optical instrument]]s such as [[telescope]]s, [[microscope]]s or in [[telescopic sight]]s ("scopes") used on [[small arm]]s and [[theodolite]]s, parallax can create problems when a reticle (or its image) is not coincident with the image plane of a target. This is because when the reticle and the target are not at the same focus, their optically corresponded distances being projected through the [[eyepiece]] are also different, and the user's eye will register the difference in parallax between the reticle and the target image (whenever eye position changes) as a relative lateral displacement on top of each other. The term ''parallax shift'' refers to the resultant apparent "floating" movements of the reticle over the target image when the user moves his/her head/eye laterally (up/down or left/right) behind the sight.<ref>{{cite web |url=https://docs.google.com/viewer?a=v&q=cache:7K5DUJIWkfoJ:viriato.net/airgunning/bfta_setup_manual.pdf+%22telescopic+sight%22+distance+between+the+barrel+and+the+sight&hl=en&gl=us&pid=bl&srcid=ADGEESh3l7c_sNAgPEc3pbi6xyOuPivRDqgtADQhQz9jsvCIPVSSrKbgSHShbhakGmiPhSO2lOO6WpI93M9BzMb0C8D3I_a1O9t48hZxEhhYpxufb3xc1hfnI2yfeqycoYyYIg5YezT-&sig=AHIEtbRaBc5RSwmyLPTzzrOTb4sGSQvTHg&pli=1 |title=Setting Up An Air Rifle And Telescopic Sight For Field Target – An Instruction Manual For Beginners, page 16 |access-date=2019-10-28}}</ref>

Some firearm scopes are equipped with a parallax compensation mechanism, which consists of a movable optical element that enables the optical system to shift the focus of the target image at varying distances into the same optical plane of the reticle (or vice versa). Many low-tier telescopic sights may have no parallax compensation because in practice they can still perform very acceptably without eliminating parallax shift. In this case, the scope is often set fixed at a designated parallax-free distance that best suits their intended usage. Typical standard factory parallax-free distances for hunting scopes are 100 yd (or 90 m) to make them suited for hunting shots that rarely exceed 300 yd/m. Some competition and military-style scopes without parallax compensation may be adjusted to be parallax free at ranges up to 300 yd/m to make them better suited for aiming at longer ranges. {{Citation needed|date=August 2011}} Scopes for guns with shorter practical ranges, such as [[airguns]], [[rimfire rifle]]s, [[shotgun]]s, and [[muzzleloader]]s, will have parallax settings for shorter distances, commonly {{cvt|50|m|yd}} for rimfire scopes and {{cvt|100|m|yd}} for shotguns and muzzleloaders. {{Citation needed|date=August 2011}} Airgun scopes are very often found with adjustable parallax, usually in the form of an adjustable objective (or "AO" for short) design, and may adjust down to as near as {{convert|3|m|yd}}.{{Citation needed|date=August 2011}}

A non-magnifying [[Reflector sight|reflector or "reflex" sight]] eliminates parallax for distant objects by using a [[Collimator#Optical collimators|collimating optic]] to image the reticle at infinity. For objects that are not infinitely far away, eye movement perpendicular to the device will cause parallax movement between the target and the reticle image that is proportional to how far the viewer's eye is off center in the cylindrical column of light created by the collimating optics.<ref name="Encyclopedia of Bullseye Pistol">{{cite web|archive-date=2011-07-08|archive-url=https://web.archive.org/web/20110708104623/http://www.bullseyepistol.com/dotsight.htm|title=Encyclopedia of Bullseye Pistol|url=http://www.bullseyepistol.com/dotsight.htm|url-status=live|website=www.bullseyepistol.com}}</ref><ref>{{cite book|title=American Rifleman|url=https://books.google.com/books?id=JhnOAAAAMAAJ|year=1944|publisher=National Rifle Association|chapter=The Reflector Sight|author=John P. Butler|page=31}}</ref> Firearm sights, such as some [[red dot sights]], try to correct for this by not imaging the reticle at infinity, but instead at a designated target distance.<ref name="Encyclopedia of Bullseye Pistol" /> 

[[Spherical aberration]] in a reflector sight can also cause the reticle's image to move with change in eye position. Some reflector sights with optical systems that compensate for off-axis spherical aberration are marketed as "parallax free".<ref>{{cite web|url=https://www.youtube.com/watch?v=UIKH5GLpL5g|title=Aimpoint's parallax-free, double lens system... AFMO.com|last=AFMOTGN|date=24 July 2008|via=YouTube|url-status=live|archive-url=https://web.archive.org/web/20160702012802/https://www.youtube.com/watch?v=UIKH5GLpL5g |archive-date=2 July 2016}}</ref><ref>{{cite web|url=http://www.ar15.com/mobile/topic.html?b=3&f=18&t=538406|title=How Aimpoints, EOTech, And Other Parallax-Free Optics Work – AR15.COM|last=AR15.COM|website=www.ar15.com}}</ref><ref>{{cite web|url=http://www.docstoc.com/docs/50408297/Gunsight---Patent-5901452|title=Gunsight – Patent 5901452 – general description of a mManginmirror system|url-status=live|archive-url=https://web.archive.org/web/20121007140016/http://www.docstoc.com/docs/50408297/Gunsight---Patent-5901452|archive-date=2012-10-07}}</ref>