Near the upper-right side of the viewfinder on your Nikon D7100 lens is a dial that enables you to adjust the viewfinder focus to accommodate your eyesight. The dial is officially known as the diopter adjustment control.
If you don’t take this step, scenes that appear out of focus through the viewfinder may actually be focused through the lens, and vice versa. Here’s how to make the necessary adjustment:
Remove the lens cap from the front of the lens.
Look through the viewfinder and press the shutter button halfway to display picture data at the bottom of the viewfinder.
In dim lighting, the built-in flash may pop up; ignore it for now and close the unit after you finish adjusting the viewfinder.
Rotate the diopter adjustment dial until the viewfinder data appears sharpest to your eye.
As you rotate the dial, the bracket-like marks in the center of the viewfinder, which are related to autofocusing, also become more or less sharp.
The Nikon manual warns you not to poke yourself in the eye as you perform this maneuver. This warning seems so obvious that you might laugh when you read it — just don't poke yourself in the eye as you perform this maneuver.