After displaying a photo in single-frame view on your Nikon D7100, as shown on the left, you can magnify it, as shown on the right. You can zoom in on still photos only, however; this feature isn’t available for movies.
Here’s how to use the magnification function:
Zoom in. Press the Qual button. You can magnify the image to a maximum of 19 to 38 times its original display size, depending on the resolution (pixel count) of the photo. Just keep pressing the button until you reach the magnification you want. (Notice the plus-sign magnifying glass symbol on the button, indicating zoom in.)
Zoom out. To zoom out to a reduced magnification, press the ISO button — the one that sports the minus-sign magnifying glass. (That little gridlike thingy next to the magnifying glass reminds you that the button also comes into play when you want to go from full-frame view to one of the thumbnail views.)
View another part of the magnified picture. When an image is magnified, a small thumbnail showing the entire image appears briefly in the lower-right corner of the screen. The yellow outline in this picture-in-picture image indicates the area that’s currently consuming the rest of the monitor space.
Use the Multi Selector to scroll the yellow box and display a different portion of the image. After a few seconds, the navigation thumbnail disappears; just press the Multi Selector in any direction to redisplay it.
Inspect faces. When you magnify portraits, the picture-in-picture thumbnail displays a white border around each face. Rotate the Sub-command dial to examine each face at the magnified view. Unfortunately, the camera sometimes fails to detect faces, especially if the subject isn't looking directly at the camera. When it works correctly, though, this is a pretty great tool for checking for closed eyes, red-eye, and, of course, spinach in the teeth.
View more images at the same magnification. While the display is zoomed, you can rotate the Main command dial to display the same area of the next photo at the same magnification.
Return to full-frame view. To return to the normal magnification level, you don’t need to keep pressing the ISO button until you’re all the way zoomed out. Instead, just press OK.