Parallax
A condition that occurs when the image of the target is not focused precisely on the reticle plane. Parallax is visible as an apparent movement between the reticle and the target when the shooter moves their head or, in extreme cases, as an out-of-focus image. Bushnell centerfire rifle scopes under 11x are factory-set parallax-free at 100 yards; rimfire and shotgun scopes at 50 yards. Scopes of 11x or more have an adjustable objective to adjust for parallax.
From Wikipedia, the free encyclopedia:
Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.[1][2] The term is derived from the Greek παράλλαξις (parallaxis), meaning "alteration". Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances. In astronomy, parallax is the only direct method by which distances to objects (typically stars) beyond the Solar System can be measured. The Hipparcos satellite has used the technique for over 100,000 nearby stars. This provides the basis for all other distance measurements in astronomy, the cosmic distance ladder. Here, the term "parallax" is the angle or semi-angle of inclination between two sightlines to the star.
Parallax also affects optical instruments such as binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields to use parallax to gain depth perception; this process is known as stereopsis.
A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a "needle" type speedometer gauge (when the needle is mounted in front of its dial scale in a way that leaves a noticeable spacing between them). When viewed from directly in front, the speed may show 60 (i.e. the needle appears against the '60' mark on the dial behind); but when viewed from the passenger seat (i.e. from an oblique angle) the needle can appear against a slightly lower or higher mark (depending on whether it is viewed from the left or from the right), because of the combined effect of the spacing and the angle of view.
Adjustable Objective
An adjustable objective is a dial around the objective end of the scope, or a knob on the left side of the turret housing that allows you to adjust your scope's parallax to a certain distance by moving these adjustments until a clear picture is perceived, and/or the marked corresponding incremental yardages are approximated to your target distance. The correct setting of an adjustable objective prevents the apparent movement between the reticle and the target when the shooter moves his head slightly off center of the rifle scope.
In conclusion
If your rifle scope is set to be "parallax free" at 100 yards you will have parallax (movement) below or above 100 yards. Slight though it may be, it's there and can and does cause focus issues. With an adjustable objective, parallax can be eliminated at any negligible distance.
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