will produce an apparent movement of the reticle "floating" over the target, known as the parallax shift. This optical effect causes aiming errors that can make a shooter miss a small target at a distance, due to actually aiming at a different spot to the assumed point of aim. It can also lead to unreliabilities when zeroing the gun. To eliminate parallax-induced aiming errors, telescopic sights can be equipped with a parallax compensation mechanism which basically consists of a movable optical element that can shift the target/reticle focus back or forward into exactly the same optical plane. There are two main methods to achieve this. By shifting the focus of the target image. This is typically achieved by making the objective lens group of the telescopic sight adjustable so the target focus can be moved into coplanarity with a fixed reticle. These models are often called adjustable objective (AO or A/O for short) models. Occasionally, a side-focus design (see below) might be used with a fixed reticle within the eyepiece, onto which the second focal plane (SFF) of the target image is shifted by an adjustable erector lens group. While side focus designs are generally considered more user-friendly than AO designs, having an SFF reticle is less ideal due to it inherently not staying true to magnification changes. By shifting the reticle position. This is typically achieved by having a movable reticle at the front of an adjustable erector lens tube, which moves forward and back in coordination with other erector lenses to shift itself into coplanarity with the first focal plane (FFP) of the target image. Because the erector tube is adjusted via an external adjustment wheel typically located on the left side of the sight's tube, these designs are called side focus (SF or S/F for short) or sidewheel models. This type of design is more costly and technically sophisticated to manufacture, but generally more favored over the AO designs by users due to better ergonomics, because unlike AO models (which need to be read from the top and adjusted by reaching all the way to the front of the sight) the SF turret setting can be conveniently read from behind and adjusted with minimal movement of the user's head. A much less commonly seen design, used exclusively in fixed-power sights, is to have a movable SFF reticle adjusted by a coaxial wheel located just in front of