Nikon D750 DSLR Camera (Body Only) - Manual (Page 368)

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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 memory card is inserted in the camera.
d6: File Number Sequence
G button
➜
A Custom Settings menu
Option Description
On
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.
Off
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.
Reset
As for On , except that the next photograph taken is assigned a
file number by adding one to the largest file number in the
current folder. If the folder is empty, file numbering is reset to
0001.
D
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 d6 ( File number sequence ) and
then either format the current memory card or insert a new memory
card.