One component of the optical system of your Nikon D7100 camera is a mirror that moves every time you press the shutter button. The small vibration caused by the action of the mirror can result in slight blurring of the image when you use a very slow shutter speed, shoot with a long telephoto lens, or take extreme close-up shots.
To cope with that issue, the D7100 offers mirror-lockup shooting, which delays opening the shutter until after the mirror movement is complete. Enable the feature by setting the Release mode to Mirror Up (labeled MUP on the Release Mode dial).
Mirror-lockup shooting requires that you press the shutter button twice to take the picture. Follow these steps:
After framing and focusing, press the shutter button all the way down to lock up the mirror.
At this point, you can no longer see anything through the viewfinder. Don’t panic — that’s normal. The mirror’s function is to enable you to see in the viewfinder the scene that the lens will capture, and mirror lockup prevents it from serving that purpose.
To record the shot, let up on the shutter button and then press it all the way down again.
If you don’t take the shot within about 30 seconds, the camera will record a picture for you automatically.
Remember that you still need to worry about moving the camera itself during the shot — even with the mirror locked up, the slightest jostle of the camera can cause blurring. In other words, situations that call for mirror lockup also call for a tripod. With the kit lens, turn off Vibration Reduction as well; with other lenses, check the manufacturer's recommendations about this issue.
Adding a remote-control shutter-release further ensures a shake-free shot — even the action of pressing the shutter button can move the camera enough to cause some slight blurring. Or you can just wait the 30 seconds needed for the camera to take the picture automatically, if your subject permits.