After you choose the Straighten filter from the Retouch menu, as shown on the left in the following figure, you see a screen similar to the one on the right, with a grid superimposed on your photo to serve as an alignment aid. Press the Multi Selector right to rotate the picture clockwise; press left to rotate counterclockwise. Each press spins the picture by about 0.25 degrees. You can achieve a maximum rotation of 5 degrees. Press OK to create the straightened copy.
To achieve this rotation magic, the camera must crop your image and enlarge the remaining area — that's why the After photo contains slightly less subject matter than the original. (The same cropping occurs if you make this kind of change in a photo editor.) The camera updates the display as you rotate the photo so that you can get an idea of how much of the original scene may be lost.