If you've used a camera in the past decade or two, you know the drill: half-press the shutter button and the camera focuses, then press the shutter all the way to take the photo. Should you find yourself second guessing the autofocus system, then you can flip a switch or go into a menu to turn off autofocus (AF) and manually focus (MF) using the lens' focus ring instead. With a single lens reflex camera you'll have a dot in the viewfinder that tells you AF or MF has been achieved successfully. If not, the Nikon D7100 has a couple of extra indicators next to the focus confirmation dot that tell you in which direction you need to turn the focus ring to achieve focus.
For some types of photography, such as close-up / macro or astrophotography, you may find yourself switching to autofocus and manual focus quite often. I'm here to tell you there is a better way: back button focus.
First, let me explain the traditional way that back button focus (BBF) has been used. I'll get back to switching between autofocus and manual focus after that.
Professional cameras often have an "AF-ON" button. When you press this button, the camera autofocuses. On a midrange camera like the D7100, there is no separate AF-ON button, but you can assign the AF-ON function to the depth of field preview button on the front of the camera (menu option f3) or the AE-L / AF-L button on the back of the camera, i.e., the infamous "back button" (menu option f4). An interesting side effect to assigning AF-ON to either of these buttons is that (half) pressing the shutter then no longer makes the camera autofocus.
So with BBF enabled, the camera will only autofocus when you press the button programmed with the AF-ON function. If you forget to do that and simply press the shutter, the camera will happily take blurry photos.
There are two additional AF settings you'll want to use together with BBF: AF-C and 3D tracking. Out of the box, the D7100 uses AF-A and AF "auto" mode. AF-S (single) means that the camera focuses once and is done, while AF-C (continuous) means that the camera keeps adjusting focus the whole time that the shutter is half-pressed or AF-ON is engaged. AF-A automatically switches between AF-S and AF-C as needed. With BBF, you'll want to use AF-C, which will give you both the AF-S and AF-C behaviors: AF-S by pressing the button and AF-C by keeping it pressed.
Having the camera automatically choose the AF sensors to be used ("auto" AF mode) is not ideal here; I prefer 3D instead. In 3D mode, you'll see one active focus point. You can select which one using the cursor pad on the back of the camera, or use "ok" to select the middle AF sensor. Usually, I prefer to use the middle one by pointing the camera to what I want to focus on and pressing the AF-ON button quickly. Then I reframe my shot and press the shutter. But if you want to track a moving subject, keep the AF-ON button pressed as you recompose your shot and the camera will try to track the subject using any of the focus points.
BBF has a number of advantages: you can take a bunch of shots in a row without the AF system possibly messing up focus and/or having to wait for autofocus to happen. You can easily focus and recompose beforehand. You get both AF-S and AF-C without having to change settings or having to rely on the camera to correctly detect what's needed.
But the most important advantage is that you get to mix autofocus and manual focus very easily with most Nikon lenses. There are basically three types of autofocus lenses:
With an M/A - M lens in BBF mode, you simply turn the focus ring to focus manually or press the AF-ON button to autofocus. If you half-press the shutter, the focus dot in the viewfinder will still tell you whether the image under the selected focus sensor is in focus or not. (Unless you use AF "auto" mode.)
Note that with these lenses, you can also adjust focus manually when not using BBF, but then you need to make sure you keep the shutter half-pressed because the AF system will reactivate if you release and then re-engage the shutter.
A lot of people swear by back button focus. I've tried it for a bit, but I kept forgetting to press the button too often, resulting in photos that weren't completely sharp. An added complication with enabling BBF is that now most people won't be able to take photos with your camera—BBF persists even in AUTO mode where pretty much all settings are automatic. So rather than keep my camera in BBF mode at all times, I set my D7100 to aperture priority and enabled BBF and then saved those settings to the U1 profile. Then, I turned BBF off again. So when I'm in AUTO, P, A, S or M mode, I'm using regular autofocus using the shutter button. But if I need to, I can easily turn on BBF by rotating the mode dial to U1,