The adjustable iris in rifle shooting is used to enhance the sight picture under various conditions. Like in a camera lens, it has the capability of increasing the “depth of focus” [often referred to as enhanced focus] This means that when the smallest practical aperature is selected, depending on conditions, there is a better chance of seeing the front sight, rear sight and target clearly at the same time. By closing the opening down as much as possible the sharpnes of the sight picture is increased. For any advanced target shooting at all, the iris is a must have. There are a number of factors that affect the diameter of the opening under various conditions. The distance from the eye to the iris affects the ammount that the iris can be closed down so the opening in the iris will be different as this distance changes. Most shooters use different eye relief from position to position.....many creep as close as two inches from the iris in the prone position and up to six inches standing. The ammount of available light in bright or dull light conditions also affects the iris opening. You will find that it is necessary, therefore, to adjust the opening almost every time you start shooting. The drill is, once you are in a comfortable shooting position, turn the adjustment ring towards the closed position slowly.....you will notice that at one point the sight picture will noticably deteriorate [it appears to suddenly go grey], at this point you have gone too far, so open it just enough to brighten the picture which will be the best setting for the existing conditions. You will eventually learn to do this very easily and try to make a habit of checking the setting often to assure it is always correct.