Nikon D5100 Reference Manual - Page 62

Focus Lock Focus lock can be used to change the composition after focusing in AF-A, AF-S, and AF-C focus modes (0 39), making it possible to focus on a subject that will not be in a focus point in the final composition. If the camera is unable to focus using autofocus (0 40), you can also focus on another subject at the same distance and then use focus lock to recompose the photograph. Focus lock is most effective when an option other than e (Auto-area AF) is selected for AF-area mode (0 42). 1 Focus. Position the subject in the selected focus point and press the shutter-release button halfway to initiate focus. Check that the in-focus indicator z (I) appears in the viewfinder. 2 Lock focus. AF-A and AF-C focus modes: With the shutter-release button pressed halfway (q), press the AE-L/AF-L button (w) to lock both focus and exposure (an AE-L icon will be displayed in the viewfinder; 0 69). Focus will remain locked while the AE-L/AFL button is pressed, even if you later remove your finger from the shutter-release button. AF-S focus mode: Focus locks automatically when the in-focus indicator appears, and remains locked until you remove your finger from the shutter-release button. Focus can also be locked by pressing the AE-L/AF-L button (see above). Shutter-release button AE-L/AF-L button 3 Recompose the photograph and shoot. Focus will remain locked between shots if you keep the shutter-release button pressed halfway (AF-S) or keep the AE-L/AF-L button pressed, allowing several photographs in succession to be taken at the same focus setting. Do not change the distance between the camera and the subject while focus lock is in effect. If the subject moves, focus again at the new distance. A The AE-L/AF-L Button Custom Setting f2 (Assign AE-L/AF-L button; 0 166) controls the behavior of the AE-L/AF-L button. 44