There are two common types of focus systems in binoculars, but a central focus system is found in most binoculars. With this system, there is a central focus knob placed between the two barrels, allowing you to adjust the focus. In a typical configuration, this knob is on the left side. This system also comes with an adjustable diopter, a smaller wheel located on the right side that lets you adjust the eyepiece for a more precise and more comfortable glassing experience.