Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 207)

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3 File Numbering Methods
Restarts the file numbering from 0001 each time the card is
replaced or a new folder is created.
When you replace the card, create a folder, or switch the target card
(such as
f 9 g ), the file numbering continues in sequence from 0001
for the images saved. This is useful if you want to organize images by
cards or folders.
If the replacement card or existing folder already contains images
recorded previously, the file numbering of the new images may continue
from the file numbering of the existing images on the card or in the
folder. If you want to save images with the file numbering starting from
0001, use a newly formatted card each time.
Resets the file numbering to 0001 or to start from file number 0001
in a new folder.
When you reset the file numbering manually, a new folder is created
automatically and the file numbering of images saved to that folder
starts, from 0001.
This is useful, for example, if you want to use different folders for the
images taken yesterday and the ones taken today. After the manual
reset, the file numbering returns to continuous or auto reset. (There will
be no manual reset confirmation dialog.)
Auto Reset
Manual Reset
0051 0001
Card A (
f
) Card B (
g
)
File numbering after replacing the card
File numbering is reset
100
0051
101
0001
Card A
File numbering after creating a folder
If the file number in folder 999 reaches 9999, shooting will not be possible
even if the card still has storage capacity. The LCD monitor will display a
message telling you to replace the card. Replace it with a new card.