Nikon D5100 Reference Manual page 8

When a photograph is taken, the camera names the file by adding one to the last file
number used. This option controls whether file numbering continues from the last
number used when a new folder is created, the memory card is formatted, or a new
memor/card is inserted in the camera.
Option
On
Off
Reset
Description
When a new folder is created, the memory card formatted, or a new memory card
inserted in the camera, file numbering continues from the last number used or from
the largest file number in the current folder, whichever is higher. If a photograph is
taken when the current folder contains a photograph numbered 9999, a new folder
will be created automatically and file numbering will begin again from 0001.
File numbering is reset to 0001 when a new folder is created, the memory card is
formatted, or a new memory card is inserted in the camera. Note that a new folder
is created automatically if a photograph is taken when the current folder contains
999 photographs.
Resets the file numbering for On to 0001 and creates a new folder with the next
photograph taken.
i_ File Number Sequence
If the current folder is numbered 999 and contains either 999 photographs or a photograph
numbered 9999, the shutter-release button will be disabled and no further photographs can
be taken. Choose Reset for Custom Setting d3 (File number sequence) and then either
format the current memory card or insert a new memory card.
In situations where the slightest camera movement can blur
pictures, select On to delay shutter release until about 1 s after
the shutter-release button is pressed and the mirror is raised.
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