Canon EOS Rebel SL3 DSLR Camera with 18-55mm Lens (Black) - Manual (Page 265)

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File Numbering Methods
The image files will be numbered from 0001 to 9999 in the order the images
are taken, then saved in a folder. You can change how the file number is
assigned.
The file number will appear on your computer in this format:
IMG_0001.JPG .
In [File numbering] under the [ 5 ] tab,
select [Numbering] , then press < 0 >.
The available settings are described below.
Select the option, then press < 0 >.
z [Continuous]: When you wish to continue the file numbering
sequence even after the card is replaced or a new folder is
created.
Even after you replace the card or create a new folder, the file
numbering continues in sequence up to 9999. This is useful when you
want to save images numbered anywhere between 0001 to 9999 on
multiple cards or in multiple folders into one folder on a computer.
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 use continuous file numbering, it is recommended
that you use a newly-formatted card each time.
0051 0052
(1)
File numbering after
replacing the card
100
0051
101
0052
File numbering after
creating a folder
Card-2 Card-1 Card-1
(1) Next sequential file number