On most DSLR cameras there is a small dial near the viewfinder; called the focus diopter. This is the focus mechanism for your viewfinder, not for your camera. This allows you to customize the image focus clarity INSIDE the viewfinder. It does not have any effect on the camera’s focusing. This is an important feature especially for those choosing to use manual focus. It also is relevant for those who wear glasses, or those who’s eyes tire.
There are a couple ways to calibrate the diopter to your eyes, but I prefer using the method with the lens on. Simply focus on an object relatively close to you (using a tripod if possible), then dial the dipoter until the image in your viewfinder looks perfectly in focus to your eyes.