The distance between the tip of the wand and the bar code is critical. The reflected light will only strike the photocell accurately from a precise distance. This distance is the nominal focal length of the wand. The size of the opening or aperture determines how precise this distance must be. Early wands used nylon or plastic tips, and these tips often were worn down from hard use.
New wands use glass tip crystals and steel tips to resist wear.
Some wand scanners have tips which may be unscrewed to replace the tip crystal. If the tip is not tightened, the wand may also become out of focus. Finally, if the bar code symbol is covered by a thick clear laminate to protect the symbol, the spacing caused by the thickness of the laminate may be too great a distance for the focal length of the wand. The aperture of the wand refers to the diameter of the opening through which the reflected light passes. The aperture determined how much of the symbol the photocell will see.