slide. With typical blade- or post-type iron sights, the shooter would center the front sight's post in the notch of the rear sight and the tops of both sights should be level. Since the eye is only capable of focusing on one focal plane at a time, and the rear sight, front sight and target are all in separate planes, only one of those three planes can be in focus. Which plane is in focus depends on the type of sight, and one of the challenges to a shooter is to keep the focus on the correct plane to allow for best sight alignment. The general advice, however, is to focus on the front sight. Due to parallax, even a tiny error in the angle of sight alignment results in a trajectory that diverges from the target on a trajectory directly relative to the distance from the target, causing the bullet to miss the target; for example, with a 10 meter air rifle shooter trying to hit the 10 ring, which is merely a diameter dot on the target at and with a diameter pellet, an error of only in sight alignment can mean a complete miss (a point of impact miss). At , that same misalignment would be magnified 100 times, giving an error of over , 1,500 times the sight misalignment. Increasing the sight radius helps to reduce eventual angular 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. With the front sight on the front end of the barrel, sight radius may be increased by moving the rear sight from the barrel onto the receiver or tang. Sights for shotguns used for shooting small, moving targets (such as clay pigeon shooting) work quite differently. The rear sight is completely discarded, and the rear reference point is provided by the correct and consistent positioning of the shooter's head. A brightly colored (generally brass or silver-colored, white, or a fluorescent shade) round bead is placed at the end of the barrel. Often, this bead will be placed along a raised, flat rib, which is usually ventilated to keep it cool and reduce mirage effects from a hot barrel. Rather than being aimed like a rifle or handgun, the shotgun is pointed with the focus always on the target, and the unfocused image of