A telescopic sight, commonly called a scope, is a device used to give additional accuracy using a point of aim for weapons such as firearms, airguns and crossbows.
Scopes are classified in terms of the optical magnification and the objective lens diameter, e.g. 10×50. This would denote 10 times magnification with a 50 mm objective lens. In general terms, larger objective lens diameters are better (collect more light and give a wider field of view), the magnification power should be chosen on the basis of the intended use. There are also adjustable objectives sights where the magnification can be changed.
Scopes come with a variety of different reticles, ranging from the traditional crosshairs to complex reticles designed to allow the shooter to estimate accurately the range to a target, to compensate for the bullet drop, and to compensate for the windage required due to crosswinds. A user can estimate the range to objects of known size, the size of objects at known distances, and even roughly compensate for both bullet drop and wind drifts at known ranges with a reticle-equipped scope.
There are two main types of reticles: Wire reticles and etched reticles
Wire reticles are the oldest type of reticles and are made out of metal wire. They are mounted in an optically appropriate position in the scopes sight's tube. Etched reticles are images of the desired reticle layout that are etched on an optic element. This optical element (lens) with the etched reticle is then mounted in the scopes sights tube as an integrated part of the optics chain of the sight. When backlit through the ocular a wire reticle will reflect incoming light and not present a black contrast reticle. An etched reticle will stay black if backlit. Etched reticles are by most considered to be a more refined solution and offer greater reticle lay out flexibility. Because of this some manufacturers can provide client designed custom reticles on special order. In the more expensive and high end contemporary scope sights etched reticles dominate the market. In cheaper scope sights wire reticles are still often mounted to avoid a rather specialized and costly production step.
The reticle may be located at the front or rear focal plane (First Focal Plane (FFP) or Second Focal Plane (SFP)) of the scope sight. On fixed power scope sights there is no significant difference, but on variable power scope sights the front plane reticle remains at a constant size compared to the target, while rear plane reticles remain a constant size to the user as the target image grows and shrinks. Front focal plane reticles are slightly more durable, but most American users prefer that the reticle remains constant as the image changes size, so nearly all modern variable power scope sights are rear focal plane designs.
Either type of reticle can be illuminated for use in low light conditions. With any illuminated reticle, it is essential that its brightness can be adjusted. A reticle that is too bright will cause glare in the operator's eye, interfering with his ability to see in low-light conditions. This is because the pupil of the human eye closes quickly upon receiving any source of light. Most illuminated reticles provide adjustable brightness settings to adjust the reticle precisely to the ambient light.
A scope sight can have several adjustment controls:
Focusing control at the ocular end of the sight - meant to obtain a sharp picture of the object and reticle.
Elevation or vertical adjustment control of the reticle.
Windage or horizontal adjustment control of the reticle.
Magnification control - meant to change the magnification by turning a ring that is generally marked with several magnification power levels.
Illumination adjustment control of the reticule - meant to regulate the brightness level of the lit parts of the reticles crosshairs.
Keith Grable is a sports enthusiast, outdoorsman and the owner of an outdoor sports and recreation website http://www.theoutdoorsports.com where a large selection of rifle scopes and accessories may be seen. |
