Nikon D5100 Reference Manual page 239

Photos are out of focus:
• AF-S or AF-I lens is not attached: use AF-S or AF-I lens or focus manually.
• Camera unable to focus using autofocus (0 40): use manual focus or focus lock (0 44, 45).
• Camera is in manual focus mode: focus manually (0 45).
Focus does not lock when shutter-release button is pressed halfway: Use AE-L/AF-L button to lock focus
when live view is off and AF-C is selected for focus mode or when photographing moving
subjects in AF-A mode (0 44).
Can not select focus point:
• e (Auto-area AF; 0 42) is selected: choose another AF-area mode.
• Press shutter-release button halfway to turn monitor off or activate exposure meters (0 28).
Subject-tracking AF not available: Select a non-monochrome Picture Control (0 91).
Can not select AF-area mode: Manual focus selected (0 39, 100).
Image size can not be changed: NEF (RAW) option selected for image quality (0 48).
Camera is slow to record photos: Turn long exposure noise reduction off (0 154).
Noise (randomly-spaced bright pixels, fog, or lines) appear in photos:
• Choose lower ISO sensitivity or turn high ISO noise reduction on (0 154).
• Shutter speed is slower than 1 s: use long exposure noise reduction (0 154).
No photo taken when remote control shutter-release button is pressed:
• Replace battery in remote control (0 207).
• Choose remote control release mode (0 35).
• Flash is charging (0 28).
• Time selected for Custom Setting c4 (Remote on duration, 0 161) has elapsed: press
camera shutter-release button halfway.
• Bright light is interfering with remote.
Beep does not sound:
• Off is selected for Custom Setting d1 (Beep; 0 161).
• Camera is in quiet shutter-release mode (0 35) or movie is being recorded (0 107).
• MF or AF-C is selected as the focus mode or subject moves when AF-A is selected (0 39).
Smudges appear in photographs: Clean front and rear lens elements. If problem persists, perform
image sensor cleaning (0 210).
Date is not imprinted on photos: An NEF (RAW) option is selected for image quality (0 48).
Sound is not recorded with movies: Microphone off is selected for Movie settings > Microphone
(0 109).
Flicker or banding appears during live view or movie recording: Choose an option for Flicker reduction
that matches the frequency of the local AC power supply (0 170).
Menu item can not be selected: Some options are not available in all modes.
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