impact a surface in a way that could potentially damage or distort the shape of the ring. Target aperture sights Target aperture sights are designed for maximum precision. The rear sight element (often called "diopter") is usually a large disk (up to 1 inch or 2.5 cm in diameter) with a small hole in the middle, of approximately or less, and is placed close to the shooter's eye. High end target diopters normally accept accessories like adjustable diopter aperture and optical filter systems to ensure optimal sighting conditions for match shooters. Typical modern target shooting diopters offer windage and elevation corrections in to increments at . Some International Shooting Sport Federation (ISSF) (Olympic) shooting events require this precision level for sighting lines, since the final score of the top competitors last shots series is expressed in tenths of scoring ring points. The complementing front sight element may be a simple bead or post, but is more often a "globe"-type sight, which consists of a cylinder with a threaded cap, which allows differently shaped removable front sight elements to be used. Most common are posts of varying widths and heights or rings of varying diameter—these can be chosen by the shooter for the best fit to the target being used. Tinted transparent plastic insert elements may also be used, with a hole in the middle; these work the same way as an opaque ring, but provide a less obstructed view of the target. High end target front sight tunnels normally also accept accessories like adjustable aperture and optical systems to ensure optimal sighting conditions for match shooters. Some high end target sight line manufacturers also offer front sights with integrated aperture mechanisms. The use of round rear and front sighting elements for aiming at round targets, like used in ISSF match shooting, takes advantage of the natural ability of the eye and brain to easily align concentric circles. Even for the maximum precision, there should still be a significant area of white visible around the bullseye and between the front and rear sight ring (if a front ring is being used). Since the best key to determining center is the amount of light passing through the apertures, a narrow, dim ring of light can actually be more difficult to work with than a larger, brighter ring. The precise sizes are quite subjective, and depend on both shooter preference and ambient lighting,