Manual Focusing in Live View Mode on Your Nikon D7100
For manual focusing in Live View mode on your Nikon D7100, set the Focus mode selector switch on the front-left side of the camera to M. If you’re using a lens that has a focus method switch, set that switch to M as well. Then turn the lens focusing ring to focus. But note a few quirks:
The focusing frame doesn’t turn green when you set focus as it does with autofocusing.
Even with manual focusing, you still see the focusing frame; its appearance depends on the current AF-area mode setting. In Face Priority mode, the frame automatically jumps into place over a face if it detects one. And if you press OK when Subject Tracking mode is enabled, the camera tries to track the subject under the frame until you press OK again.
You can press the Qual button to check focus in manual mode just as you can during autofocusing. After magnifying the scene, press the Multi Selector to scroll the display to view a different area.
Press the ISO button to reduce the magnification level.
If you’re using an AF-S lens, you actually don't need to move the camera's Focus mode selector switch to the M position to focus manually — simply setting the lens switch to the M position tells the camera that you're about to focus manually. Just be sure that your lens falls in this category, or you can damage the lens and the camera by not adjusting both switches.