Canon EOS 90D Advanced User's Manual page 509

Auto Reset
When you wish to restart the fi le numbering from 0001 each
time the card is replaced or a new folder is created.
When you replace the card or create a folder, the fi le numbering restarts
from 0001 for the new 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 fi le numbering of the new images may continue from the fi le
numbering of the existing images on the card or in the folder. If you want
to save images with the fi le numbering starting from 0001, use a newly
formatted card each time.
File numbering after
replacing the card
Card 1
0051
(1) File numbering is reset
Manual Reset
When you wish to reset the fi le numbering to 0001 or to start
from fi le number 0001 in a new folder.
When you reset the fi le numbering manually, a new folder is created
automatically and the fi le numbering of images saved to that folder starts
from 0001.
This is useful, for example, if you want to use diff erent folders for the images
taken yesterday and the ones taken today.
Card 2
0001
(1)
File Numbering Methods
File numbering after
creating a folder
Card 1
100 101
0051 0001
509