How to Change the Focusing Method on Your Nikon D7100
Assuming that your lens supports autofocusing when mounted on the Nikon D7100, familiarize yourself with these two controls, which set the focusing method to manual or autofocusing:
Lens focus-mode switch: Assuming that your lens offers autofocusing as well as manual focusing, it has a switch that you use to choose between the two options. On the 18–105mm kit lens shown, the switch has an M setting for Manual focus and an A setting for Autofocus. Other lenses may offer different switches, so check your lens instruction guide for specifics.
Focus-mode selector: Also shown, this switch sets the camera itself to manual focusing (M) or autofocusing (AF).
Manual focusing is fairly simple: Just rotate the focus ring on the lens to bring your subject into focus. The placement and appearance of the focus ring depend on the lens; the figure shows you the one on the 18–105mm kit lens.