<P> Parallax affects sights in many ways . On sights fitted to small arms, bows in archery, etc. the distance between the sighting mechanism and the weapon's bore or axis can introduce significant errors when firing at close range, particularly when firing at small targets . This difference is generally referred to as "sight height" and 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 . 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 be useful from 50m to 200m without further adjustment . </P> <P> In optical sights parallax can exist when the reticle is not coincident with the focal plane of the target image, and the term parallax shift refers to the apparent movement of the reticle in relationship to the target when the user moves his / her head laterally behind the sight (up / down or left / right), i.e. it is an error where the reticle does not stay aligned with the sight's own optical axis . </P> <P> In optical instruments such as telescopes, microscopes, or in telescopic sights used on small arms and theodolites, the error occurs when the optics are not precisely focused: the reticle will appear to move with respect to the object focused on if one moves one's head sideways in front of the eyepiece . Some firearm telescopic sights are equipped with a parallax compensation mechanism which basically consists of a movable optical element that enables the optical system to project the picture of objects at varying distances and the reticle crosshairs pictures together in exactly the same optical plane . Telescopic sights may have no parallax compensation because they can perform very acceptably without refinement for parallax with the sight being permanently adjusted for the distance that best suits their intended usage . Typical standard factory parallax adjustment distances for hunting telescopic sights are 100 yd or 100 m to make them suited for hunting shots that rarely exceed 300 yd / m . Some target and military style telescopic sights 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 . Scopes for rimfires, shotguns, and muzzleloaders will have shorter parallax settings, commonly 50 yd / m for rimfire scopes and 100 yd / m for shotguns and muzzleloaders . Scopes for airguns are very often found with adjustable parallax, usually in the form of an adjustable objective, or AO . These may adjust down as far as 3 yards (2.74 m). </P> <P> Non-magnifying reflector or "reflex" sights have the ability to be theoretically "parallax free ." But since these sights use parallel collimated light this is only true when the target is at infinity . At finite distances eye movement perpendicular to the device will cause parallax movement in the reticle image in exact relationship to eye position in the cylindrical column of light created by the collimating optics . Firearm sights, such as some red dot sights, try to correct for this via not focusing the reticle at infinity, but instead at some finite distance, a designed target range where the reticle will show very little movement due to parallax . Some manufactures market reflector sight models they call "parallax free," but this refers to an optical system that compensates for off axis spherical aberration, an optical error induced by the spherical mirror used in the sight that can cause the reticle position to diverge off the sight's optical axis with change in eye position . </P>

Who demonstrated a method to calculate the distance of celestial bodies