Canon EOS 40D - Manual (Page 72)

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The file number is like the frame number on a roll of film. The captured
images are assigned a sequential file number from 0001 to 9999 and
saved in one folder. You can also change how the file number is
assigned.
The file number will appear on the personal computer in this format:
IMG_0001.JPG.
1
Select [File numbering].
Under the [5] tab, select [File
numbering], then press <0>.
2
Select the file numbering method.
Turn the <5> dial to select the
desired method, then press <0>.
File numbering continues even after CF card replacement
Even after you replace the CF card, the file numbering continues in
sequence up to 9999. This is convenient when you want to save the
images numbered anywhere between 0001 to 9999 in one folder in your
personal computer.
If the replacement CF card contains images recorded previously, the file
numbering of the new images might continue from the file numbering of
the existing images in the card. If you want to use continuous file
numbering, you should use a newly-formatted CF card each time.
3 File Numbering Methods
Continuous
XXX-0051
CF card -1
XXX-0052
CF card -2
File numbering after replacing the CF card
Next sequential file number