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A diopter sight is an aperture sight component used to assist the aiming of ranged weapons, mainly firearms, airguns, and crossbows. Diopters function to precisely align the shooter's eye with the front sight and the target, while also producing beneficial optical effects for accurate aiming. A diopter must be paired with a complementing front sight element to obtain a usable sighting line. Diopter sights used for modern target shooting allow for very fine windage and elevation adjustments, moving the impact point on the order of less than five millimeters at a range of 100 metres. High end diopters typically accept accessories to aid the shooter's ability to see the target clearly.
Diopters are found in particular as rear sight elements on . Diopter and globe sighting setups are commonly used in ISSF rifle shooting events.
The small diopter viewing opening ensures the shooter's eye is very precisely and consistently centered behind the diopter sight. The diopter sight is easy to use and usually allows for very accurate aiming, because a relative long sighting line can be used. A long sighting line helps to reduce eventual angle 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.
The parallax distance of a diopter sight is effectively adjusted to be the same as the sight distance. For example, with a distance of between the front and rear sight, the sighting system is effectively parallax adjusted to a distance of in front of the rear sight. However, since the diopter hole is small, the aiming eye will be relatively well centered, and the parallax error will be relatively small in practice. There will be almost no parallax error if the eye and front globe sight are near perfectly centered through the diopter. In comparison, the parallax error on aperture sights with larger openings can increase quickly if the eye is not well centered in the rear sight. A parallax error of just at corresponds to an impact change of at .
A rear sight with a larger aperture than is not strictly a diopter sight, but nonetheless is still often (incorrectly) referred to as such. With larger aperture sights the shooter must make a conscious effort to center the eye in the rear sight for precise aiming. A true diopter sight (aperture below 1.2 mm) however has the advantage that the shooter does not have to concentrate on eye and rear sight alignment for precision aiming, and therefore the sighting process is reduced to only aligning the front sight to the target.
Aperture sights (both diopter and non-diopter) require being placed close to the aiming eye, while open sights have to be placed at least 30 cm away from the eye to in order to appear sharp.
High-end target shooting diopters normally accept accessories like:
For optimal aiming and comfort the shooter should focus the aiming eye on the front sighting element. To avoid eye fatigue and improve balance the non-aiming eye should be kept open. The non-aiming eye can be blocked from seeing distractions by mounting a semi-transparent occluder to the diopter.
For 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 of the employed components are quite subjective, and depend on both shooter preference and ambient lighting, which is why target rifles come with easily replaceable front sight inserts, and adjustable aperture mechanisms.
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