rifles and compact "tactical"-style semi-automatic rifles used among the law enforcement, home defense and practical shooting enthusiasts crowd. Specifications Optical parameters Telescopic sights are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters. Those parameters are: Magnification – The ratio of the focal length of the eyepiece divided into the focal length of the objective gives the linear magnifying power of telescopes. A magnification factor of 10, for example, produces an image as if one were 10 times closer to the object. The amount of magnification depends upon the application the telescopic sight is designed for. Lower magnifications lead to less susceptibility to shaking. A larger magnification leads to a smaller field of view. Objective lens diameter – The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters. Field of view – The field of view of a telescopic sight is determined by its optical design. It is usually notated in a linear value, such as how many meters (feet) in width will be seen at , or in an angular value of how many degrees can be viewed. Exit pupil – Telescopic sights concentrate the light gathered by the objective into a beam, the exit pupil, whose diameter is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris – for a youthful dark-adapted human eye about 7 mm, reducing with age. If the cone of light streaming out of the eyepiece is larger than the pupil it is going into, any light shining outside the pupil is considered "wasted" in terms of providing visual information. However, a larger exit pupil makes it easier to put the eye where it can receive the light: anywhere in the large exit pupil cone of light will do. This ease of placement helps avoid vignetting, which is a darkened or obscured view that occurs when the light path is partially blocked. And, it means that the image can be quickly found which is important when aiming at game animals that move rapidly. A narrow exit pupil telescopic sight may also be fatiguing because the instrument must be held exactly in place in front of the eyes to provide a useful image. Finally, many